In runexecutor.py, send SIGTERM to the process
if the soft time limit is reached.
This might help verifiers that do not implement a soft time limit
themselves to still terminate gracefully.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13538 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c