torture: Allow four-digit repetition numbers for --configs parameter

In a clear-cut case of "not thinking big enough", kvm.sh limits the
multipliers for torture-test scenarios to three digits.  Although this is
large enough for any single system that I have ever run rcutorture on,
it does become a problem when you want to use kvm-remote.sh to run as
many instances of TREE09 as fit on a set of 20 systems with 80 CPUs each.

Yes, one could simply say "--configs '800*TREE09 800*TREE09'", but this
commit removes the need for that sort of hacky workaround by permitting
four-digit repetition numbers, thus allowing "--configs '1600*TREE09'".

Five-digit repetition numbers remain off the menu.  Should they ever
really be needed, they can easily be added!

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