Reduce the number of iterations for cgroups_unittest.
Reduce the number of iterations for cgroups_unittest.

cgroups_unittest is currently our slowest test, taking 40 seconds, compared
with repository_unittest, which takes 35 seconds. Tune down the number of
iterations to speed it up so that it isn't the slowest test and doesn't bog
down run_tests.

BUG=none
TEST=Run it. It now takes 26 seconds instead of 40.

Change-Id: I288711a27ac5c182d51996b0ee5f9edd1667eed9
Reviewed-on: https://gerrit.chromium.org/gerrit/39368
Commit-Ready: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

zhang0137/chromite,zhang0137/chromite,coreos/chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite