hypothesis: Fix or ignore deprecation warnings since version 5.6

Since version 5.6, hypothesis warns on `@given` + function-scoped fixtures.
So turns concerned swh-web fixtures from function-scoped to session-scoped
(there is strictly no difference in tests execution result after that change)
and ignore some warnings related to external function-scoped fixtures that
are safe to use with @given (client from pytest-django and mocker from
pytest-mock).