selftests: Speedup functional..multiplex_test

multiplex_test uses sleeptest to check it supplies the values correctly.
This patch uses passtest instead.

For now it uses the same multiplex file and in `test_run_mplex_timeout`
still uses sleeptest, but there is FIXME message to use envtest once
it supports params passing (another pull request)

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>