Tests flexible to auth method

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At the moment, each auth method (Service key path / service key contents / user) has a separate class with tested functions

I think we could make this much simpler and more flexible - have a pytest fixture that generates a GBQConnector, and use that in each function. Then we can test on whatever auth method is available (and set a minimum if preferred: #72)

Separately, there is a lot of setup & teardown code that could be simplified through fixtures. And some manual test_ids that could be automated through pytest

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This was committed to master a while ago. Plenty of improvements to the test infra are still possible, but the bulk of this is in