gnu.tests: Check gnu lister
This triggers a gnu listing and checks the output from the scheduler.
For this, a new swh-listers fixture got added. This allows to retrieve a test ready lister from its name (e.g gnu). Those listers have then access to a scheduler fixture to be able to check the lister's output in the scheduler backend.
Related T2023
Test Plan
tox
Migrated from D2074 (view on Phabricator)