Parallel unit tests in one process shared the process-wide count of live
document script workers, so unrelated tests together reached the 16-worker
cap and failed at random with "document script worker limit reached".
Under cfg(test) each test thread now has its own count against the same cap;
a permit releases the count it was taken from. The production cap and its
process-wide count are unchanged.
mtlcl410
795bc1144b