glib/tests/unix: Mostly pass O_CLOEXEC to g_unix_pipe_open()
Using FD_CLOEXEC here is now deprecated. Keep exactly one call with FD_CLOEXEC, in test_pipe_fd_cloexec().
Using FD_CLOEXEC here is now deprecated. Keep exactly one call with FD_CLOEXEC, in test_pipe_fd_cloexec().