[AIRFLOW-2801] Skip test_mark_success_no_kill in PostgreSQL on CI
[AIRFLOW-2801] Skip test_mark_success_no_kill in PostgreSQL on CI

See mailing list thread "Flaky test case:
test_mark_success_no_kill".

Closes #3642 from
tedmiston/test_mark_success_no_kill-postgresql

lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow