Ensure that airflow modules are not imported too early in conftest (#…

…17779)

If someone adds `import airflow` or any submodule in to conftest it will
cause airflow.settings and airflow.configuration to be imported and
initialized before we set the test env vars -- this leads to _some_
tests failing in bizarre ways.

tests/conftest.py

@@ -25,6 +25,7 @@
 
 # We should set these before loading _any_ of the rest of airflow so that the
 # unit test mode config is set as early as possible.
+assert "airflow" not in sys.modules, "No airflow module can be imported before these lines"
 tests_directory = os.path.dirname(os.path.realpath(__file__))
 
 os.environ["AIRFLOW__CORE__DAGS_FOLDER"] = os.path.join(tests_directory, "dags")