avocado.job: Do not forward SIGINT to subprocesses

This ends up making the subprocesses to receive the
SIGINT multiple times, leading to the multiprocess
logging to misbehave.

Also, remove the ctrl+c test, given that we couldn't
figure out a way to properly reproduce a SIGINT passed
to the process group of the running avocado instance.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>