test_flatten_same_pcollections (apache_beam.transforms.ptransform_test.PTransformTest) does not work in Streaming VR suite on Dataflow

Per investigation in https://issues.apache.org/jira/browse/BEAM-8877, the test times out and was recently added to VR test suite.

liumomo315, I will sickbay this test for streaming, could you please help triage the failure?

Thank you!