Introduce Trigger Test Harness

As mentioned in https://docs.google.com/document/d/1Xp-YBf87vLTduYSivgqWVEMjYUmkA-hyb4muX3KRl08/edit# we should build on the processing-time clock introduced in FLINK-3646 and add a testing harness for triggers. The harness should allow inputting elements, updating watermark, updating processing-time and firing timers. The output of the trigger as well as state managed by the partitioned state abstraction should be observable to verify correct trigger behavior.

Then, we should add tests for all triggers.