Admin: Improve moto usage, simplifying and speeding up tests

[bblommers]

Hi there! Moto maintainer here. I had a look at the tests for s3fs, and realized that there is a cleaner way to use the Moto library for your usecase.

The ThreadedMotoServer starts an instance of MotoServer in the background, achieving the same result as calling moto_server s3 in a subprocess. So the behaviour is the same, it just hides the nasty details like calling subprocess.Popen, and having to manually call requests.get to verify whether the server is ready.

The second change is to turn the s3_base fixture into a module-scoped fixture. The ThreadedMotoServer will now only be created once per module (file), meaning that the tests are executed much faster (~3mins vs ~5 mins).

Because we do expect a clean slate in between tests, I added reset_s3_fixture - calling the reset-API in Moto deletes all existing state before every test.

[martindurant]

That's great, thanks for getting in touch and contributing, and general thanks for moto's existence :)