docs: added documentation on testing tools (#8939)

- Added documentation on how to use Medusa's tools from the `medusa-test-utils` package to create integration and unit tests.
- Added a manual reference on the `medusaIntegrationTestRunner` and `moduleIntegrationTestRunner` functions. Since the typings in the source code aren't very informative, I opted for a manual reference shedding light on the important bits.

Closes DOCS-852

www/apps/book/app/debugging-and-testing/testing-tools/modules-tests/module-example/page.mdx

@@ -0,0 +1,70 @@
+export const metadata = {
+  title: `${pageNumber} Example: Integration Tests for a Module`,
+}
+
+# {metadata.title}
+
+In this chapter, find an example of writing an integration test for a module using the [moduleIntegrationTestRunner utility function](../page.mdx).
+
+## Write Integration Test for Module
+
+Consider a `hello` module with a `HelloModuleService` that has a `getMessage` method:
+
+```ts title="src/modules/hello/service.ts"
+import { MedusaService } from "@medusajs/utils"
+import MyCustom from "./models/my-custom"
+
+class HelloModuleService extends MedusaService({
+  MyCustom,
+}){
+  getMessage(): string {
+    return "Hello, World!"
+  }
+}
+
+export default HelloModuleService
+```
+
+To create an integration test for the method, create the file `src/modules/hello/__tests__/service.spec.ts` with the following content:
+
+```ts title="src/modules/hello/__tests__/service.spec.ts"
+import { moduleIntegrationTestRunner } from "medusa-test-utils"
+import { HELLO_MODULE } from ".."
+import HelloModuleService from "../service"
+import MyCustom from "../models/my-custom"
+
+moduleIntegrationTestRunner<HelloModuleService>({
+  moduleName: HELLO_MODULE,
+  moduleModels: [MyCustom],
+  resolve: "./modules/hello",
+  testSuite: ({ service }) => {
+    describe("HelloModuleService", () => {
+      it("says hello world", () => {
+        const message = service.getMessage()
+
+        expect(message).toEqual("Hello, World!")
+      })
+    })
+  }
+})
+```
+
+You use the `moduleIntegrationTestRunner` function to add tests for the `hello` module. You have one test that passes if the `getMessage` method returns the `"Hello, World!"` string.
+
+---
+
+## Run Test
+
+Run the following command to run your module integration tests:
+
+```bash npm2yarn
+npm run test:modules
+```
+
+<Note title="Tip">
+
+If you don't have a `test:modules` script in `package.json`, refer to the [Medusa Testing Tools chapter](../../page.mdx#add-test-commands).
+
+</Note>
+
+This runs your Medusa application and runs the tests available in any `__tests__` directory under the `src` directory.

www/apps/book/app/debugging-and-testing/testing-tools/modules-tests/page.mdx

@@ -0,0 +1,101 @@
+export const metadata = {
+  title: `${pageNumber} Write Tests for Modules`,
+}
+
+# {metadata.title}
+
+In this chapter, you'll learn about the `moduleIntegrationTestRunner` utility function and how to use it to write integration tests for a module's main service.
+
+## moduleIntegrationTestRunner Utility
+
+The `moduleIntegrationTestRunner` utility function is provided by the `medusa-test-utils` package to create integration tests for a module. The integration tests run on a test Medusa application with only the specified module enabled.
+
+For example, assuming you have a `hello` module, create a test file at `src/modules/hello/__tests__/service.spec.ts`:
+
+```ts title="src/modules/hello/__tests__/service.spec.ts"
+import { moduleIntegrationTestRunner } from "medusa-test-utils"
+import { HELLO_MODULE } from ".."
+import HelloModuleService from "../service"
+import MyCustom from "../models/my-custom"
+
+moduleIntegrationTestRunner<HelloModuleService>({
+  moduleName: HELLO_MODULE,
+  moduleModels: [MyCustom],
+  resolve: "./modules/hello",
+  testSuite: ({ service }) => {
+    // TODO write tests
+  }
+})
+```
+
+The `moduleIntegrationTestRunner` function accepts as a parameter an object with the following properties:
+
+- `moduleName`: The name of the module.
+- `moduleModels`: An array of models in the module. Refer to [this section](#write-tests-for-modules-without-data-models) if your module doesn't have data models.
+- `resolve`: The path to the model relative to the `src` directory.
+- `testSuite`: A function that defines the tests to run.
+
+The `testSuite` function accepts as a parameter an object having the `service` property, which is an instance of the module's main service.
+
+<Note title="Tip">
+
+The type argument provided to the `moduleIntegrationTestRunner` function is used as the type of the `service` property.
+
+</Note>
+
+The tests in the `testSuite` function are written using [Jest](https://jestjs.io/).
+
+---
+
+## Pass Module Options
+
+If your module accepts options, you can set them using the `moduleOptions` property of the `moduleIntegrationTestRunner`'s parameter.
+
+For example:
+
+```ts
+import { moduleIntegrationTestRunner } from "medusa-test-utils"
+import HelloModuleService from "../service"
+
+moduleIntegrationTestRunner<HelloModuleService>({
+  moduleOptions: {
+    apiKey: "123"
+  },
+  // ...
+})
+```
+
+---
+
+## Write Tests for Modules without Data Models
+
+If your module doesn't have a data model, pass a dummy model in the `moduleModels` property.
+
+For example:
+
+```ts
+import { moduleIntegrationTestRunner } from "medusa-test-utils"
+import HelloModuleService from "../service"
+import { model } from "@medusajs/utils"
+
+const DummyModel = model.define("dummy_model", {})
+
+moduleIntegrationTestRunner<HelloModuleService>({
+  moduleModels: [DummyModel],
+  // ...
+})
+```
+
+---
+
+### Other Options and Inputs
+
+Refer to [this reference in the Learning Resources documentation](!resources!/test-tools-reference/moduleIntegrationTestRunner) for other available parameter options and inputs of the `testSuite` function.
+
+---
+
+## Database Used in Tests
+
+The `moduleIntegrationTestRunner` function creates a database with a random name before running the tests. Then, it drops that database after all the tests end.
+
+To manage that database, such as changing its name or perform operations on it in your tests, refer to the [references in the Learning Resources documentation](!resources!/test-tools-reference/moduleIntegrationTestRunner).