Moved the test/bench runner path behind an optional flag

So now instead of needing:

  ./scripts/test.py ./runners/test_runner test_dtree

You can just do:

  ./scripts/test.py test_dtree

Or with an explicit path:

  ./scripts/test.py -R./runners/test_runner test_dtree

This makes it easier to run the script manually. And, while there may be
some hiccups with the implicit relative path, I think in general this will
make the test/bench scripts easier to use.

There was already an implicit runner path, though only if the test suite
was completely omitted. I'm not sure that would ever have actually
been useful...

---

Also increased the permutation field size in --list-*, since I noticed it
was overflowing.

scripts/test.py

@@ -1457,15 +1457,14 @@ if __name__ == "__main__":
 
     # test flags
     test_parser = parser.add_argument_group('test options')
-    test_parser.add_argument(
-        'runner',
-        nargs='?',
-        type=lambda x: x.split(),
-        help="Test runner to use for testing. Defaults to %r." % RUNNER_PATH)
     test_parser.add_argument(
         'test_ids',
         nargs='*',
         help="Description of tests to run.")
+    test_parser.add_argument(
+        '-R', '--runner',
+        type=lambda x: x.split(),
+        help="Test runner to use for testing. Defaults to %r." % RUNNER_PATH)
     test_parser.add_argument(
         '-Y', '--summary',
         action='store_true',