conftest: Declare swh.core pytest_plugin
As the other pytest plugin are declared.
The pytest.ini modification is required because it autoloads (as in swh-model!137 (closed) for example).
I tried to stop swh.core from autoloading its fixture but it's apparently more complex than anticipated [1]. I don't want to plunge in there just yet so here goes.
Test Plan
tox
Migrated from D4182 (view on Phabricator)