Replace stateful hypothesis strategies by pytest fixtures

swh-web define hypothesis strategies to provide test inputs from a sample archive populated with small git repositories.

While it enables to deeply test webapp code, it has several drawbacks:

• tests execution are slow as multiple examples are generated by hypothesis
• tests data are generated at module loading time which is not a good practice
• some strategies are stateful which does not play well with hypothesis to get reproducible examples

We should replace those strategies by simple pytest fixtures instead.

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Migrated from T3603 (view on Phabricator)