GBKs on unbounded pcolls with global windows and no triggers should fail

This, according to "4.2.2.1 GroupByKey and unbounded PCollections" in https://beam.apache.org/documentation/programming-guide/.

If you do apply GroupByKey or CoGroupByKey to a group of unbounded PCollections without setting either a non-global windowing strategy, a trigger strategy, or both for each collection, Beam generates an IllegalStateException error at pipeline construction time.

Example where this doesn't happen in Python SDK: https://stackoverflow.com/questions/60623246/merge-pcollection-with-apache-beam

I also believe that this unit test should fail, since test_stream is unbounded, uses global window, and has no triggers.

  def test_global_window_gbk_fail(self):
    with TestPipeline() as p:
      test_stream = TestStream()
      _ = p | test_stream | GroupByKey()