Make parserTests.php more git bisect-friendly

Make parserTests.php more git bisect-friendly

Now the following commands can track down parser test regressions (if
you're using git svn, of course):

  git bisect start bad-rev good-rev
  git bisect run maintenance/parserTests.php --regex 'Test name'

Just type those in (replacing "bad-rev" and "good-rev" with revision
ids, and 'Test name' with a regex that uniquely matches the regressing
test's name), and git will magically find the first bad revision for you
in a couple of minutes.  (Then run git bisect reset to get back to
normal.)

All I changed to make this possible was changing the error return code
from 255 (-1) to 1.  git bisect treats 1-124 and 126-127 as error codes,
but for some reason doesn't like 128-255.