[AIRFLOW-XXX] Speed up RBAC view tests (#4162)
[AIRFLOW-XXX] Speed up RBAC view tests (#4162)

Not re-creating the FAB app ones per test functions took the run time of
the TestAirflowBaseViews from 223s down to 53s on my laptop, _and_ made
it only print the deprecation warning (fixed in another PR already open)
once instead of 10+ times.
owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow