This regress checks that 400 singals are evenly distributed over 4

This regress checks that 400 singals are evenly distributed over 4
threads.  It works well on idle 4 CPU machines, but with load or
fewer cores the distribution of the threads' signal handlers is
worse.  Reduce the limit of signals that each thread must receive
from 75 to 50.  This should prevent random test fails.
from Christian Ludwig