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.
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]);
}