TestStream.testDiscardingMode 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/testDiscardingMode/

```

Expected: iterable with items ["firstPane", "alsoFirstPane", "onTimePane"] in any order
but:
not matched: "alsoFinalLatePane"
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.testDiscardingMode(TestStreamTest.java:222)

```

Imported from Jira [BEAM-12039](https://issues.apache.org/jira/browse/BEAM-12039). Original Jira may contain additional context.
Reported by: kenn.