Rerun fragile tests only with status == 'fail'

Sometimes fragile tests can be skipped or disabled. Such tests were
considered as not passed and test-run rerun them which is obviously
redundant. This patch fixes the issue.

lib/worker.py

@@ -352,7 +352,7 @@ def run_loop(self, task_queue, result_queue):
                 )
             retries_left = self.suite.fragile_retries()
             # let's run till short_status became 'pass'
-            while short_status != 'pass' and retries_left >= 0:
+            while short_status in [None, 'fail'] and retries_left >= 0:
                 self.restart_server()
                 # print message only after some fails occurred
                 if short_status == 'fail':