[Flake][DF IT tests] job ##### terminated in state UNKNOWN but did not return a failure reason.

Dataflow integration tests fail regularly failing to properly detect job status. Usually error looks like:

java.lang.RuntimeException: Dataflow job 2019-03-21_05_08_46-17069693240682025803 terminated in state UNKNOWN but did not return a failure reason. at org.apache.beam.runners.dataflow.TestDataflowRunner.run(TestDataflowRunner.java:134)

We have several flakes like this a week. (More statistics might be good to add)

Sample log.
https://builds.apache.org/job/beam_PostCommit_Java11_ValidatesRunner_PortabilityApi_Dataflow/17/testReport/junit/org.apache.beam.sdk.transforms.windowing/WindowTest/testTimestampCombinerDefault/