testsuite: rehabilitate mux test

make everything run in a single thread so that it is not an
unintentional test of thread interleaving semantics.  Update the
result because now some output transitions consistently happen one
servo cycle earlier than before.

The test now passes (200x) on my machine with PREEMPTY_RT when
merged into the rtos branch.  On the same test machine, the old test
failed somewhere between 1-in-10 and 1-in-50 of the time.