config/tests: inject NMDBusManager for tests in NMDevice stub

We don't need the bus for the tests and the manager may warn when it
is not available.

  $ (cd src/tests/config/; env -i DBUS_SYSTEM_BUS_ADDRESS=meow ./test-config)

  /config/parse-error: OK
  /config/no-auto-default: NetworkManager-Message: <info>  Could not connect to the system bus; only the private D-Bus socket will be available.
  /bin/sh: line 5: 29997 Trace/breakpoint trap   ${dir}$tst
  FAIL: test-config

This reverts commit 6994454461 for the
most part. It's not sufficient to disable logging warnings. Creating
a DBus Manager might affect the system in undesired ways.

(cherry picked from commit 5040a8a851)

src/tests/config/test-config.c

@@ -28,6 +28,7 @@
 #include "nm-test-device.h"
 #include "nm-fake-platform.h"
 #include "nm-logging.h"
+#include "nm-dbus-manager.h"
 
 #include "nm-test-utils.h"
 
@@ -289,6 +290,13 @@ main (int argc, char **argv)
 {
 	nmtst_init_assert_logging (&argc, &argv);
 
+	/* Initialize the DBus manager singleton explicitly, because it is accessed by
+	 * the class initializer of NMDevice (used by the NMTestDevice stub).
+	 * This way, we skip calling nm_dbus_manager_init_bus() which would
+	 * either fail and/or cause unexpected actions in the test.
+	 * */
+	nm_dbus_manager_setup (g_object_new (NM_TYPE_DBUS_MANAGER, NULL));
+
 	nm_fake_platform_setup ();
 
 	g_test_add_func ("/config/simple", test_config_simple);