Move all celery-related fixtures to the swh.scheduler pytest plugin

This allows us to reuse these fixtures in other modules without brittle
swh.scheduler.tests.conftest star imports.

Unfortunately, we can't really override pytest fixtures from one plugin to
another. We therefore reimplement the fixtures provided by celery, inlining the
static configuration and renaming them to our names in the process.

This also adds a backwards-compatibility import from pytest_plugin to conftest,
to allow old users of the conftest fixtures to keep working.