rcutorture: Allow repetition factors in Kconfig-fragment lists

Although it is currently possible to run the same test in parallel,
'--config "TINY01 TINY01 TINY01"' can get a bit verbose, especially
if you want to run 48 instances of TINY01 in parallel.  This commit
therefore allows prefixing the Kconfig fragment with a repeat count,
for example, '--config "48*TINY01"' to run 48 instances in parallel.
At least assuming that you have 48 CPUs and also gave '--cpus 48'.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>