chore: re-work generating unit golden files

[chanseokoh]

Now we can completely remove the rules_bazel folder.

To generate a golden file, all we have to do is to run a unit test, because those unit tests are written to generate and save a golden. For example,

  @Test
  public void generateGrpcServiceStubClass_createBatchingCallable() {
    ...
    // Generate a golden while running this test.
    Utils.saveCodegenToFile(this.getClass(), "GrpcLoggingStub.golden", visitor.write());
    ...
    Assert.assertCodeEquals(goldenFilePath, visitor.write());
  }

I think this is better than my previous attempt: #911

[codecov]

Codecov Report

Merging #912 (81f1bbe) into main (d210aa1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #912   +/-   ##
=======================================
  Coverage   87.84%   87.84%           
=======================================
  Files         153      153           
  Lines       16046    16046           
  Branches     1166     1166           
=======================================
  Hits        14095    14095           
  Misses       1607     1607           
  Partials      344      344           

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[chanseokoh]

FTR: this is the line running SingleJUnitTestRunner.

{test_runner_path} $@ is roughly

java -cp ... com.google.api.generator.test.framework.SingleJUnitTestRunner <SomeUnitTestClass>

as shown here. All it does is to run unit tests (i.e., @Test methods) in <SomeUnitTestClass>. (Unit tests in the class should be designed to save a golden file.)