Change hermetic build integration test to golden test

[JoeWang1127]

Currently, the integration test (hermetic_build/library_generation/tests/integration_tests.py) uses a branch (chore/test-hermetic-build) in 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.