[w32] test_shm_mq test suite permanently burns connections slots

On a Windows or other EXEC_BACKEND build, the following eventually gets
failures because all, or all but one, max_connections slot is consumed:
 for run in `seq 1 100`; do make -C contrib/test_shm_mq installcheck; done

When I use max_connections=40, it fails on the sixth iteration.  Only the six
basic processes are actually running at that time.

Thanks,
nm

-- 
Noah Misch
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