tests: Change --test_dirs to --test-dirs.

tests/run-tests

@@ -111,7 +111,7 @@ def run_tests(pyb, tests):
 def main():
     cmd_parser = argparse.ArgumentParser(description='Run tests for Micro Python.')
     cmd_parser.add_argument('--pyboard', action='store_true', help='run the tests on the pyboard')
-    cmd_parser.add_argument('-d', '--test_dirs', nargs='*', help='input test directories (if no files given)')
+    cmd_parser.add_argument('-d', '--test-dirs', nargs='*', help='input test directories (if no files given)')
     cmd_parser.add_argument('files', nargs='*', help='input test files')
     args = cmd_parser.parse_args()