use direct executor to deflake tests

[m-trieu]

MoreExecutors.directExecutor()/directExecutorService runs all tasks on the calling thread (w/o offloading to another thread for async work) and calls to submit and execute will block until the submitted task returns (i.e Runnable.run()).

Use this in test implementations of ChannelCache and FanOutStreamingEngineWorkerHarness to prevent threads waiting on each other. The old implementation seems to work locally but in the test runner environment has increased in flakiness.

Flakiness is referenced in https://github.com/apache/beam/issues/28957

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