tests: Mock logind’s Inhibit method

This is needed to allow gnome-shell to start up during its unit tests,
as it now calls `Inhibit` to make saving screen time information
race-free.

See
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3643#note_2367420.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8185
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4314>

src/tests/dbusmock-templates/logind.py

@@ -158,6 +158,11 @@ def GetSessionByPID(self, pid):
     session_path = f'{MAIN_OBJ}/session/{self.preferred_session_id}'
     return session_path
 
+@dbus.service.method(MANAGER_IFACE, in_signature='ssss', out_signature='h')
+def Inhibit(self, what, who, why, mode):
+    # Return an arbitrary FD
+    return os.open('/dev/null', os.O_RDONLY)
+
 def create_session(self, host_bus):
     session_id = None
     seat_id = None