CombineTest.testGlobalCombineWithDefaultsAndTriggers is broken

The test is not run because `p.run()` is not called. When `p.run()` is added, the test fails.

Kenn, I suspect this is because it's using triggers in batch, which obviously is not guaranteed to work.

Please investigate!

https://github.com/apache/incubator-beam/blob/master/sdks/java/core/src/test/java/com/google/cloud/dataflow/sdk/transforms/CombineTest.java#L373

Failed tests:
CombineTest.testGlobalCombineWithDefaultsAndTriggers:391
Expected: iterable over ["2: true", "1: false"] in any order
but: No item matches: "1: false" in ["2: true"]

Tests run: 30, Failures: 1, Errors: 0, Skipped: 0