Merge branch '2322-fflush-test-dbus' into 'master'

gtestdbus: Flush stdout and stderr before forking a monitor process

Closes #2322

See merge request GNOME/glib!1950

gio/gtestdbus.c

@@ -251,6 +251,16 @@ watcher_init (void)
           g_assert_not_reached ();
         }
 
+      /* flush streams to avoid buffers being duplicated in the child and
+       * flushed by both the child and parent later
+       *
+       * FIXME: This is a workaround for the fact that watch_parent() uses
+       * non-async-signal-safe API. See
+       * https://gitlab.gnome.org/GNOME/glib/-/issues/2322#note_1034330
+       */
+      fflush (stdout);
+      fflush (stderr);
+
       switch (fork ())
         {
         case -1: