argparsing: Parser: allow to forward prog to argparse

Ref: https://github.com/pytest-dev/pytest-xdist/pull/388.

testing/test_parseopt.py

@@ -24,6 +24,12 @@ class TestParser(object):
         out, err = capsys.readouterr()
         assert err.find("error: unrecognized arguments") != -1
 
+    def test_custom_prog(self, parser):
+        """Custom prog can be set for `argparse.ArgumentParser`."""
+        assert parser._getparser().prog == os.path.basename(sys.argv[0])
+        parser.prog = "custom-prog"
+        assert parser._getparser().prog == "custom-prog"
+
     def test_argument(self):
         with pytest.raises(parseopt.ArgumentError):
             # need a short or long option