Simplify test init

Summary:
mimetype: Simplify test init

Bootstrap in-mem idx storage

Test the in-mem idx storage.

fossology_license: simplify test init.

re-fix mimetype

ctags: simplify test init.

ctags+language: simplify test init.

Simplify tests in swh/indexer/tests/test_metadata.py.

Reviewers: ardumont

Reviewed By: ardumont

Subscribers: swh-public-ci

Differential Revision: https://forge.softwareheritage.org/D725