TrigValSteering: Produce backtrace on timeout

Before killing a process due to timeout, produce a backtrace and write it to the log file. Uses $ROOTSYS/etc/gdb-backtrace.sh as suggested by @fwinkl in ATR-20263.

Backtrace is produced for the given PID and all children processes. The main motivation was that the PID corresponds to a shell opened by subprocess.Popen so we need to get the child anyway. Since we're already doing that, a recursive retrieval may be more useful for debugging timeouts.