tests: fix leak in mainloop test

Properly unref a pair of GSources in the unix-fd mainloop test.

valgrind was reporting these as 'still reachable' before (possibly due
to some residual pointers somewhere in memory), but when running with
G_DEBUG=cleanup they were properly reported as leaked.

glib/tests/mainloop.c

@@ -1273,7 +1273,9 @@ test_unix_fd_source (void)
   g_assert (in && out);
 
   g_source_destroy (out_source);
+  g_source_unref (out_source);
   g_source_destroy (in_source);
+  g_source_unref (in_source);
   close (fds[1]);
   close (fds[0]);
 }