Add helper function to ensure loader visit/visit-status are as expected
This bootstraps a new swh.loader.tests.common module to declare common helper functions we can reuse for all loaders.
(I wonder if putting this directly in swh/loader/tests/init.py is not enough?)
Following a discussion with @olasd, i preferred to open a new diff to decrease the size of the original one [1]
- [1] Related to !382 (closed)#inline-22935
Test Plan
tox
Migrated from D3321 (view on Phabricator)