scripts/sanitycheck: Precompile test-time regular expressions

The Harness handlers for tests were parsing the realtime stream out of
qemu pipes by recompiling and executing every regex for every line (!)
of output from the simulator.  That's a significant CPU load, and it's
(1) in a separate thread not tracked by the JOBS limit and (2)
happening at the worst possible time and contending with the qemu
process for host CPU cycles that it needs to hit its (real world)
timer targets on time.

Compile them just once, please.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>