Reduce boilerplate in unit tests

Thanks to #96, we can reduce the amount of boilerplate in the unit test suite considerably.

In general, tests now work as follows:

If the test does not require access to any mock adaptors (RabbitAdaptor, LookupResultProcessor, etc.), simply use the client fixture:
```
def test_something(client):
client.get(...)
```
If the test requires access to an adaptor such as the RabbitAdaptor in order to change its behavior or make assertions, use the appropriate mock adaptor fixture (or create a mock manually in the test function), and then pass it to the ResourceTestBase._test_client method as before:
```
def test_something_with_adaptor(mock_rabbit_adaptor):
client = self._test_client(rabbit_adaptor=mock_rabbit_adaptor)
mock_rabbit_adaptor.assert_called_with(...)
```
Most tests no longer need to create a test client manually, but it may also still be necessary if you want to pass in some custom configuration values:
```
def test_something_with_custom_config():
client = self._test_client(extra_config={"ENABLE_AUTH": True})
...
```

There's still quite a bit of duplicated code throughout the unit test suite that should be refactored to Pytest fixtures, but this is a good start.

ssl-hep / ServiceX_App