Add assertCodeEquals to TestUtils and swap code to use it

[`assertCodeEquals()`](https://github.com/grpc/grpc-java/blob/master/testing/src/main/java/io/grpc/internal/testing/AbstractTransportTest.java#L813) (also [here](https://github.com/grpc/grpc-java/blob/5d025ed6ae329061dd17c828045234be73282697/android-interop-testing/app/src/main/java/io/grpc/android/integrationtest/InteropTester.java#L730)) is really helpful for debugging test failures from CI environments because it provides more information about the Status versus `assertEquals(Code, Code)`. Issues like #1513 would have enough information to better understand what went wrong.