tests: Add a pytest plugin introducing the swh_objstorage fixture
Add a pytest plugin enabling to create a new object storage instance
for tests using the swh_objstorage
fixture.
The type of object storage to create is determined by the configuration
returned by the swh_objstorage_config
fixture.
Use these new fixtures to simplify a bit some tests implementation.
Related to swh-scrubber#4694.