tests: make the kafka_server_base fixture a function scoped one

this actually speeds up tests quite a bit (preventing a 30s timeout when
the fixture is actually reused several times in a test session).

swh/journal/pytest_plugin.py

@@ -182,7 +182,7 @@ def kafka_server(
     return kafka_server_base
 
 
-@pytest.fixture(scope="session")
+@pytest.fixture(scope="function")
 def kafka_server_base() -> Iterator[str]:
     """Create a mock kafka cluster suitable for tests.