docs: fixes to testing tool guides (#8981)

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

@@ -78,7 +78,9 @@ import { moduleIntegrationTestRunner } from "medusa-test-utils"
 import HelloModuleService from "../service"
 import { model } from "@medusajs/utils"
 
-const DummyModel = model.define("dummy_model", {})
+const DummyModel = model.define("dummy_model", {
+  id: model.id().primaryKey()
+})
 
 moduleIntegrationTestRunner<HelloModuleService>({
   moduleModels: [DummyModel],

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

@@ -15,7 +15,7 @@ Medusa provides a `medusa-test-utils` package with utility tools to create integ
 To use the `medusa-test-utils` package, install it as a `devDependency`:
 
 ```bash npm2yarn
-npm install --save-dev medusa-test-utils
+npm install --save-dev medusa-test-utils@preview
 ```
 
 ---
@@ -88,11 +88,11 @@ You now have two commands:
 
 - `test:integration:http` to run integration tests (for example, for API routes and workflows) available under the `integration-tests/http` directory.
 - `test:integration:modules` to run integration tests for modules available in any `__tests__` directory under `src/modules`.
-- `test:unit` to run unit tests in any `__tests` directory under the `src` directory.
+- `test:unit` to run unit tests in any `__tests__` directory under the `src` directory.
 
 <Note>
 
-Unit tests aren't covered by Medusa's testing tools.
+Medusa provides utility tools for integration tests only. You can write unit tests using Jest.
 
 </Note>