Event time timer not firing with TestStream

https://github.com/andrewrjones/beam-test-stream-timer/blob/master/src/test/java/com/andrewjones/beam/TimerTest.java

Unclear whether it is a DirectRunner problem or a core SDK problem. For now, assuming it is DirectRunner.

Imported from Jira [BEAM-3341](https://issues.apache.org/jira/browse/BEAM-3341). Original Jira may contain additional context.
Reported by: kenn.

Start with the referenced src/test/java/com/andrewrjones/beam/TimerTest.java and reproduce the event-time timer failure using TestStream. Trace whether the behavior belongs to DirectRunner or the core SDK. Done means identifying the responsible component and making the event-time timer fire correctly with a regression test.

Written by the indexing model from the issue text.