TestAdmissionController.test_release_backend is flaky

Error Message
assert 'NumCompletedBackends: 1 (1)' in 'Query (id=c341f85f14c3d981:5827484900000000):\n  DEBUG MODE WARNING: Query profile created while running a DEBUG buil... - OptimizationTime: 372.020ms\n           - PeakMemoryUsage: 528.00 KB (540672)\n           - PrepareTime: 39.002ms\n'  +  where 'Query (id=c341f85f14c3d981:5827484900000000):\n  DEBUG MODE WARNING: Query profile created while running a DEBUG buil... - OptimizationTime: 372.020ms\n           - PeakMemoryUsage: 528.00 KB (540672)\n           - PrepareTime: 39.002ms\n' = <bound method BeeswaxConnection.get_runtime_profile of <tests.common.impala_connection.BeeswaxConnection object at 0x5323510>>(<tests.common.impala_connection.OperationHandle object at 0x5462cd0>)  +    where <bound method BeeswaxConnection.get_runtime_profile of <tests.common.impala_connection.BeeswaxConnection object at 0x5323510>> = <tests.common.impala_connection.BeeswaxConnection object at 0x5323510>.get_runtime_profile  +      where <tests.common.impala_connection.BeeswaxConnection object at 0x5323510> = <test_admission_controller.TestAdmissionController object at 0x51f5fd0>.client
Stacktrace
custom_cluster/test_admission_controller.py:1338: in test_release_backends
    assert "NumCompletedBackends: 1 (1)" in self.client.get_runtime_profile(handle)
E   assert 'NumCompletedBackends: 1 (1)' in 'Query (id=c341f85f14c3d981:5827484900000000):\n  DEBUG MODE WARNING: Query profile created while running a DEBUG buil... - OptimizationTime: 372.020ms\n           - PeakMemoryUsage: 528.00 KB (540672)\n           - PrepareTime: 39.002ms\n'
E    +  where 'Query (id=c341f85f14c3d981:5827484900000000):\n  DEBUG MODE WARNING: Query profile created while running a DEBUG buil... - OptimizationTime: 372.020ms\n           - PeakMemoryUsage: 528.00 KB (540672)\n           - PrepareTime: 39.002ms\n' = <bound method BeeswaxConnection.get_runtime_profile of <tests.common.impala_connection.BeeswaxConnection object at 0x5323510>>(<tests.common.impala_connection.OperationHandle object at 0x5462cd0>)
E    +    where <bound method BeeswaxConnection.get_runtime_profile of <tests.common.impala_connection.BeeswaxConnection object at 0x5323510>> = <tests.common.impala_connection.BeeswaxConnection object at 0x5323510>.get_runtime_profile
E    +      where <tests.common.impala_connection.BeeswaxConnection object at 0x5323510> = <test_admission_controller.TestAdmissionController object at 0x51f5fd0>.client