Cleanup external transform tests

Currently apache_beam/transforms/external_test.py has several entry points, sometimes called directly, sometimes via nosetest, sometimes with parameters passed via arguments or via environment variables, and the logic is not always clear to follow (either within the test, or via the several gradle targets that reference it). We should really let this file be a unit test, and create a different script (sharing a common library if needed) in our integration tests.

This was the root cause of BEAM-8302 .

Imported from Jira [BEAM-8305](https://issues.apache.org/jira/browse/BEAM-8305). Original Jira may contain additional context.
Reported by: robertwb.