BigQuery test refinement #1712

In data_sources/test_bigquery.py we set:

os.environ[consts.ENV_DIRECTORY_VAR] = f"gs://{TEST_BUCKET}/integration_tests/"

I imagine the intention was that this takes effect only for specific tests in this module but it ends up affecting all tests. We should look into refining this so the environment change is only in effect where intended.

This is not currently doing any harm so is low priority.