PG: dmap-quiescence-test hangs with one process

The dmap-quiescence-test example presented in Ch 8 has a subtle defect. Specifically, the quiescence loop currently lacks a call that guarantees user-level progress, which means that under "pathological" conditions this loop can livelock. This condition arises with high probability on many systems when running with a single rank (although it might vary due to GASNet implementation details), causing a single rank to livelock.

The fix is to add an explicit call to upcxx::progress() in the loop.

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