torture: Don't kill gdb sessions

The rcutorture scripting will do a "kill -9" on any guest OS that exceeds
its --duration by more than a few minutes, which is very valuable when
bugs result in hangs.  However, this is a problem when the "hang" was due
to a --gdb debugging session.

This commit therefore refrains from killing the guest OS when a debugging
session is in progress.  This means that the user must manually kill the
kvm.sh process group if a hang really does occur.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>