TestDataflowRunner should log explicitly when it is interruped

Right now, one is left to deduce that a test was interrupted, where a well placed log line around https://github.com/apache/beam/blob/master/runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/TestDataflowRunner.java#L163 would make it obvious. It won't be spammy, as things are about to terminate if this ever happens.

Imported from Jira [BEAM-6893](https://issues.apache.org/jira/browse/BEAM-6893). Original Jira may contain additional context.
Reported by: kenn.

Open runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/TestDataflowRunner.java around line 163 and trace how interruption is handled. Add an explicit log at that point so the interrupted test is obvious before termination. Done means the interruption path emits a clear, non-spammy log message.

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