Create a test system bus and use it to run all the tests, add a mock
SensorsProxy (via dbusmock template) server that implements the
net.hadess.SensorProxy interface.
To make testing easier, the service is created on request of a proxy for
it, whose lifetime controls the mock service lifetime as well.
This is done using a further mock service that is used to manage the
others, using python-dbusmock to simplify the handling.
Add basic tests for the orientation manager.
As per the usage dbusmock, we're now launching all the tests under such
wrapper, so that local dbus environment won't ever considered, and
there's no risk that it may affect the tests results both locally and in
CI.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1233>
