add standalone test runner

tested with:
• Python 3.9.2 on Debian
• Python 2.5.5 on MirBSD

test.py

+#!/usr/bin/env python3
+# ~*~ coding: utf-8 ~*~
+#-
+# Copyright © 2021
+#       mirabilos <m@mirbsd.org>
+#
+# Provided that these terms and disclaimer and all copyright notices
+# are retained or reproduced in an accompanying document, permission
+# is granted to deal in this work without restriction, including un‐
+# limited rights to use, publicly perform, distribute, sell, modify,
+# merge, give away, or sublicence.
+#
+# This work is provided “AS IS” and WITHOUT WARRANTY of any kind, to
+# the utmost extent permitted by applicable law, neither express nor
+# implied; without malicious intent or gross negligence. In no event
+# may a licensor, author or contributor be held liable for indirect,
+# direct, other damage, loss, or other issues arising in any way out
+# of dealing in the work, even if advised of the possibility of such
+# damage or existence of a defect, except proven that it results out
+# of said person’s immediate fault when using the work as intended.
+
+#import os.path
+#import sys
+#sys.path.append(os.path.join(os.path.dirname(__file__), 'src'))
+
+from test.test_musicxml import *
+from test.test_parser import *
+from test.test_playlist import *
+
+unittest.main()