integration-test: Check polkit denial

integration-test: Check polkit denial

Add a test case for a polkit-denied operation: we try to mount an internal disk
as user "nobody". Technically the system polkit configuration could allow this,
but let's hope that this is not the case (this is an integration test, after
all).

We need to supply our own D-BUS configuration for this to allow nobody to
access our temporary fake system bus. Thus drop the usage of Gio.TestDBus and
replace it with calling dbus-launch.