Creates test request helper for API endpoints

Separates common utils into the medusa-core-utils package.
Sets up a testing environment where mocked models/services/etc. can be placed in
__mocks__ folder within its corresponding directory. The mocks will
automatically be registered in a awilix container only used for testing.

packages/medusa/medusa-config.js

@@ -1,10 +1,3 @@
 module.exports = {
-  plugins: [
-    {
-      resolve: `mrbl-payment-stripe`,
-      options: {
-        stripeApiKey: "12345",
-      },
-    },
-  ],
+  plugins: [],
 }