Improve gs-specific tests.

  * Refactored gs tests into their own gs module.
  * Fixed misspelling of harness.
  * Added gs credential detection and unittest skip for all gs tests.
Added tests for GCS versioning feature and fixed a couple bugs that surfaced.
Renamed two util functions to better names and be clearer in versioning test comments.
Merge pull request #1206 from jterrace/gs-test-fixes

Refactored GCS tests and added tests for GCS versioning feature.