torture: Add torture.sh torture-everything script

Although tailoring a specific set of kvm.sh runs has served rcutorture
testing well over many years, it requires a relatively distraction-free
environment, which is not always available.  This commit therefore
adds a prototype torture.sh script that by default tortures pretty much
everything the rcutorture scripting is designed to torture, and which
can be given command-line arguments to take a more focused approach.

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