ci/bare-metal: Fix detection of "POWER_GOOD not seen in time" fails

We were only reading from the CPU serial, not EC, so we'd never notice
these sources of job timeouts.  I couldn't find a cleaner solution, so I
spawned two threads to do the blocking reads from our serial line fifos
and merge them together in a single queue to read.

Closes: #3470
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6529>
1 file changed