loader.mercurial: Increase coverage

As per my suggestion [1], transformed the loader verifier code into tests.

• loader_verifier: Transform the verifier into tests

• tests_tasks: Add scenario to check tasks are fine

• test_converters: Add coverage on converters

• Add scenario for archive mercurial loader (success + failure)

• [1] https://forge.softwareheritage.org/swh/devel/swh-environment#1411

Related swh-environment#1411 (closed) Related T1439 Related swh-environment#1443 (closed)

Test Plan

tox

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Migrated from D802 (view on Phabricator)