tests: Add a pytest plugin introducing the swh_objstorage fixture

Add a pytest plugin enabling to create a new object storage instance for tests using the swh_objstorage fixture.

The type of object storage to create is determined by the configuration returned by the swh_objstorage_config fixture.

Use these new fixtures to simplify a bit some tests implementation.

Related to swh-scrubber#4694 (closed).

Jenkins job DOBJS/gitlab-builds #166 succeeded .
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mentioned in merge request swh-scrubber!64 (merged)

approved this merge request

added 25 commits

• dca7a0b0...1ef64997 - 24 commits from branch swh/devel:master
• a8e0b4eb - tests: Add a pytest plugin introducing the swh_objstorage fixture

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Jenkins job DOBJS/gitlab-builds #171 succeeded .
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merged