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

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Migrated from D3321 (view on Phabricator)