loader.hg: Initialize and refactor deprecated tests to use pytest fixture

This starts the refactoring of the loader-mercurial tests to use pytest. Taking the same path we did for loader-svn [1] (one test at a time).

Hopefully, at some point, we will be able to reuse those tests for !25 (closed), maybe?

Note: This will allow to remove the unit-test based tests scaffolding from the loader-core (once done).

Related to T2481

• [1] swh-loader-svn#2462 (closed)

Test Plan

tox

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Migrated from D3440 (view on Phabricator)