TestStream.testLateDataAccumulating gives wrong answer on FlinkRunner

https://ci-beam.apache.org/job/beam_PostCommit_Java_ValidatesRunner_Flink_PR/358/testReport/junit/org.apache.beam.sdk.testing/TestStreamTest/testLateDataAccumulating/

```

Expected: iterable with items [<1>, <2>, <3>, <4>, <5>] in any order
but: not matched: <-1>
at
org.apache.beam.sdk.testing.PAssert$PAssertionSite.capture(PAssert.java:174)
at org.apache.beam.sdk.testing.PAssert.that(PAssert.java:416)
at
org.apache.beam.sdk.testing.PAssert.that(PAssert.java:408)
at org.apache.beam.sdk.testing.TestStreamTest.testLateDataAccumulating(TestStreamTest.java:129)

```

Imported from Jira [BEAM-12038](https://issues.apache.org/jira/browse/BEAM-12038). Original Jira may contain additional context.
Reported by: kenn.
This issue has child subcomponents which were not migrated over. See the original Jira for more information.