TimestampCombiner.EARLIEST is not respected when elements are late.

In ReduceFnRunner#onTrigger, because late elements don't set a hold, we do not output with the timestamp that would be expected by the TimestampCombiner.

Imported from Jira [BEAM-2262](https://issues.apache.org/jira/browse/BEAM-2262). Original Jira may contain additional context.
Reported by: tgroh.

Start at ReduceFnRunner#onTrigger and trace how late elements are handled when TimestampCombiner.EARLIEST is configured. Compare the emitted timestamp with the combiner's expected timestamp and add coverage for this late-element path. Done means late elements produce output with the timestamp required by TimestampCombiner.EARLIEST.

Written by the indexing model from the issue text.