sleep(100000) command from test_shell_commandline.py can hang around and cause test_metrics_are_zero to fail

03:25:47 [gw6] PASSED shell/test_shell_commandline.py::TestImpalaShell::test_cancellation 
...
03:27:01 verifiers/test_verify_metrics.py:34: in test_metrics_are_zero
03:27:01     verifier.verify_metrics_are_zero()
03:27:01 verifiers/metric_verifier.py:47: in verify_metrics_are_zero
03:27:01     self.wait_for_metric(metric, 0, timeout)
03:27:01 verifiers/metric_verifier.py:62: in wait_for_metric
03:27:01     self.impalad_service.wait_for_metric_value(metric_name, expected_value, timeout)
03:27:01 common/impala_service.py:135: in wait_for_metric_value
03:27:01     json.dumps(self.read_debug_webpage('rpcz?json')))
03:27:01 E   AssertionError: Metric value impala-server.mem-pool.total-bytes did not reach value 0 in 60s