g_test_trap_fork: don't blow away the SIGCHLD handler

Not sure why it was doing this, but it's not necessary (all of glib's
tests pass fine without it), and it breaks tests that try to use
g_spawn_sync() or GChildWatchSource after doing a g_test_trap_fork().

https://bugzilla.gnome.org/show_bug.cgi?id=679683