tests: Turn snapshot* hypothesis strategies into pytest fixtures

swh/web/tests/api/test_utils.py

swh/web/tests/api/views/test_identifiers.py

swh/web/tests/api/views/test_snapshot.py

swh/web/tests/browse/test_snapshot_context.py

swh/web/tests/browse/views/test_identifiers.py

swh/web/tests/common/test_archive.py

swh/web/tests/common/test_identifiers.py

swh/web/tests/common/test_middlewares.py

swh/web/tests/conftest.py

swh/web/tests/misc/test_badges.py

swh/web/tests/strategies.py