Add a bin/managetests script to run this from the git repository

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

bin/managetests

+#! /usr/bin/python3
+
+import os
+import sys
+
+
+# Prefer local modules over any system-installed ones to ensure that running a
+# Git version from any current working directory does not have unexpected
+# behaviour.
+parent = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+if os.path.exists(os.path.join(parent, 'managetests', '__init__.py')):
+    sys.path.insert(0, parent)
+
+
+if __name__ == '__main__':
+    import managetests
+    managetests.main.cli()