Support running tests under `HARNESS_OPTIONS=j10`. GH Issue #12.

Previously, all work was done in a scratch dir at

  $DIST/t/x/

And each test was using a notifcation on

  $DIST/lib/
  $DIST/x/

So when each test added new files to scratch dir, all tests listening
for notificaton saw the event, and saw file creations they had not
anticipated.

This patch resolves that by using File::Temp to create a tempdir for
each test, and to only listen on that subdir

  $DIST/lib/
  $DIST/x/{TEMPDIRPREFIX}/
Merge pull request #13 from kentfredric/multiharness

Support running tests under `HARNESS_OPTIONS=j10`. GH Issue #12.