tests/dbus-runner: Forward logind methods when not in KVM

This means one can run meta-dbus-runner.py effectively mocking
everything relevant except logind itself, meaning one can run from a TTY
and get permission to mode set etc, while still mocking things like
gsd-color, colord, etc.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2618>

src/tests/logind_helpers.py

@@ -1,6 +1,21 @@
 import dbus
 import os
 import re
+import sys
+
+
+host_system_bus_connection = None
+
+
+def ensure_system_bus(address):
+    global host_system_bus_connection
+
+    if host_system_bus_connection is None:
+        bus = dbus.bus.BusConnection(address)
+        host_system_bus_connection = bus
+
+    return host_system_bus_connection
+
 
 def open_file_direct(major, minor):
     sysfs_uevent_path = '/sys/dev/char/{}:{}/uevent'.format(major, minor)
@@ -19,3 +34,14 @@ def open_file_direct(major, minor):
     unix_fd = dbus.types.UnixFd(fd)
     os.close(fd)
     return (unix_fd, False)
+
+
+def call_host(address, object_path, interface, method, typesig, args):
+    bus = ensure_system_bus(address)
+
+    return bus.call_blocking('org.freedesktop.login1',
+                             object_path,
+                             interface,
+                             method,
+                             typesig,
+                             args)