Fix cygwin noise

Three of the tests living in test/parallel/taskPar/sungeun/barrier/ (basic, reuse,
and split-phase) have been sporadically timing out on cygwin since we started
testing the full test suite.  This is because their problem size is too large for
cygwin to handle: the tests always complete when run outside of the testing
system, usually after 3-6 minutes.  After examining our code, I believe it is
the use of atomics for a large number of tasks that causes the slow down -
atomics with a smaller number of tasks performs just fine, and syncs/singles
with the same number of tasks also completes in a reasonable amount of
time.  The point of the benchmarks does not seem to be dependent on the
number of tasks involved, so dialing down the problem size seems to be the
best solution.