Tests: add tst_signond using QtDBusMock

The python-dbusmock project [1] and its Qt bindings [2] allow easy and complete testing of D-Bus components. We take these facilities into use to test signond in isolation, by providing a mock version of signon-ui and by directly calling its D-Bus methods, without using libsignon-qt.

These new tests are enabled only if the libqtdbusmock package is found.

[1] https://pypi.python.org/pypi/python-dbusmock

[2] https://launchpad.net/libqtdbusmock