doc: Add testing guides to DEVELOPMENT.md (#1747)

DEVELOPMENT.md

@@ -38,4 +38,29 @@ for gapic-generator-java's Bazel build.
 
     ```sh
     mvn fmt:format
-    ```
+    ```
+
+## Testing guide
+There are 4 layers of testing in this repo:
+1. Traditional unit tests. These tests usually focus on testing one very specific test case at a time. For example, [MethodTest](https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/src/test/java/com/google/api/generator/gapic/model/MethodTest.java) 
+2. Golden unit tests. These tests start with a test proto, we create different test cases in the proto, load the proto in unit tests and save the generated file as golden files. For example, [routing header proto](https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/src/test/proto/explicit_dynamic_routing_header_testing.proto) -> Load in the [unit test](https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/grpc/GrpcServiceStubClassComposerTest.java#L56-L64) -> Saved as [golden](https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/grpc/goldens/GrpcRoutingHeadersStub.golden)    
+3. Showcase integration tests. They test the generated library's behavior against a mock server, see [README.md](https://github.com/googleapis/sdk-platform-java/blob/main/showcase/README.md) of showcase module for details.
+4. Golden integration tests. These tests generate a full client library with real protos from googleapis, for example, golden integration tests for [compute](https://github.com/googleapis/sdk-platform-java/tree/main/test/integration/goldens/compute). However, we _should_ stop adding new ones, and rely on golden unit tests and showcase tests.
+
+Based on where the code changes are, we should add different tests, in general
+
+- If the changes are in `gax` or `api-common` only, you _must_ add traditional unit tests, you _may_ add showcase integration tests if you are modifying a public setting.
+- If the changes are in `gapic-generator-java` only, showcase integration tests are most likely not needed
+  - If it is in the `composer` module, you _must_ add golden unit tests, you _may_ add traditional unit tests for better coverage and easier testing.  
+  - If it is in the `parser` module, you _may_ add traditional unit tests with a test proto. For example, [routing_rule_parser_testing](https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/src/test/proto/routing_rule_parser_testing.proto) that is only used for testing [RoutingRuleParser](https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/RoutingRuleParser.java). 
+  - If it is in `other modules(ast, model, writer etc.)`, you _must_ add traditional unit tests, you _may_ add golden unit tests to easily see the changes.
+- If the changes are in both `gax` and `gapic-generator-java`, you _must_ add all test layers, including traditional unit tests, golden unit tests and showcase integration tests. 
+
+### How to run tests?
+
+See the [Unit Tests](https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/DEVELOPMENT.md#unit-tests) section in gapic-generator-java/DEVELOPMENT.md for how to run traditional and golden unit tests.
+
+See [Running the Integration Tests](https://github.com/googleapis/sdk-platform-java/blob/main/showcase/README.md#running-the-integration-tests) and [Update the Golden Showcase Files](https://github.com/googleapis/sdk-platform-java/blob/main/showcase/README.md#update-the-golden-showcase-files) sections in showcase/README.md for how to run and update showcase integration tests.
+
+See the [Integration Tests](https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/DEVELOPMENT.md#integration-tests) section in gapic-generator-java/DEVELOPMENT.md for how to run golden integration tests.
+