At various places we sort our D-Bus paths. For example,
server sorts them before exporting them on D-Bus.

Server knows well, that a lot of these paths are build
by attaching an incrementing number as last component.
It looks nicer to sort by this number, instead of strictly
lexical with strcmp().

Note that this handles the cases correctly where paths have
different prefixes, or where they don't end with a number.

Like g_utf8_collate(), which complains about NULL strings.

tools/test-networkmanager-service.py is our NetworkManager stub server.

NetworkManager uses libnm(-core) heavily, for example to decide whether
a connection verifies (nm_connection_verify()) and for normalizing
connections (nm_connection_normalize()).

If the stub server wants to mimic NetworkManager, it also must use these
function. Luckily, we already can do so, by loading libnm using python
GObject introspection.

We already correctly set GI_TYPELIB_PATH search path, so that the
correct libnm is loaded -- provided that we build with introspection
enabled.

We still need to gracefully fail, if starting the stub server fails.
That requries some extra effort. If the stub server notices that
something is missing, it shall exit with status 77. That will cause
the tests to g_test_skip().