[sdk-platform-java] Change hermetic build integration test to golden test

Currently, the integration test (`hermetic_build/library_generation/tests/integration_tests.py`) uses a branch (`chore/test-hermetic-build`) in [google-cloud-java](https://github.com/googleapis/google-cloud-java) to verify the generation result.

This setting makes it not obvious on how to change the golden files.

We should copy the files from google-cloud-java as golden files to verify the integration test.