run-tests: allow option parser to be extended

This patch moves the OptionParser population into its own function so
consumers may modify the OptionParser before arguments are evaluated.
This will allow consumers to add custom options, set different defaults,
etc.

tests/run-tests.py

@@ -133,7 +133,7 b' def parselistfiles(files, listtype, warn'
         f.close()
     return entries
 
-def parseargs(args):
+def getparser():
     parser = optparse.OptionParser("%prog [options] [tests]")
 
     # keep these sorted
@@ -210,6 +210,10 b' def parseargs(args):'
     for option, (envvar, default) in defaults.items():
         defaults[option] = type(default)(os.environ.get(envvar, default))
     parser.set_defaults(**defaults)
+
+    return parser
+
+def parseargs(args, parser):
     (options, args) = parser.parse_args(args)
 
     # jython is always pure
@@ -1176,8 +1180,9 b' def runtests(options, tests):'
 testtypes = [('.py', pytest, '.out'),
              ('.t', tsttest, '')]
 
-def main(args):
-    (options, args) = parseargs(args)
+def main(args, parser=None):
+    parser = parser or getparser()
+    (options, args) = parseargs(args, parser)
     os.umask(022)
 
     checktools()