test: make flaky pummel test more reliable

Increase the number of iterations from 1e4 to 1e5.  Makes the test pass
for me locally when previously it would fail 9 out of 10 times because
the running time was not enough to smooth away the outliers.

Fixes: https://github.com/nodejs/node/issues/8744
PR-URL: https://github.com/nodejs/node/pull/9241


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: not-an-aardvark <not-an-aardvark@users.noreply.github.com>