feature_graphene_passive is a floating test

[scravy]

Occasionally feature_graphene_passive fails on my machine (Ubuntu 18.10, gcc 8.3.0).

This is the error reported:

2019-05-27T11:59:38.868000Z TestFramework.utils (ERROR): wait_until() failed. Predicate: (['        wait_until(lambda: get_msg() is not None, timeout=10)\n'], 290)
2019-05-27T11:59:38.868000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "/home/scravy/Github/scravy/unit-e/test/functional/test_framework/test_framework.py", line 203, in main
    self.run_test()
  File "/home/scravy/Github/scravy/unit-e/test/functional/feature_graphene_passive.py", line 69, in run_test
    self.test_non_synced_mempool(relay)
  File "/home/scravy/Github/scravy/unit-e/test/functional/feature_graphene_passive.py", line 97, in test_non_synced_mempool
    graphene_tx = relay.wait_for((b'graphenetx', block_hash)).graphene_tx
  File "/home/scravy/Github/scravy/unit-e/test/functional/feature_graphene_passive.py", line 290, in wait_for
    wait_until(lambda: get_msg() is not None, timeout=10)
  File "/home/scravy/Github/scravy/unit-e/test/functional/test_framework/util.py", line 370, in wait_until
    raise AssertionError("Predicate {} not true after {} seconds".format(predicate_source, timeout))
AssertionError: Predicate (['        wait_until(lambda: get_msg() is not None, timeout=10)\n'], 290) not true after 10 seconds
2019-05-27T11:59:38.921000Z TestFramework (INFO): Stopping nodes
2019-05-27T11:59:39.124000Z TestFramework (WARNING): Not cleaning up dir /tmp/test_runner_U⋮_🟟_20190527_135356/feature_graphene_passive_147
2019-05-27T11:59:39.124000Z TestFramework (ERROR): Test failed. Test logging available at /tmp/test_runner_U⋮_🟟_20190527_135356/feature_graphene_passive_147/test_framework.log
2019-05-27T11:59:39.124000Z TestFramework (ERROR): Hint: Call /home/scravy/Github/scravy/unit-e/test/functional/combine_logs.py '/tmp/test_runner_U⋮_🟟_20190527_135356/feature_graphene_passive_147' to consolidate all logs

[scravy]

Just realised this has already been reported: #1055