torture: Permit running of experimental torture types

This commit weakens the checks of the kvm.sh script's --torture parameter
and the kvm-recheck.sh script's parsing so that experimental torture tests
may be created without updating these two scripts.  The changes required
are to the appropriate Makefile and Kconfig file, plus a directory
whose name begins with "X" must be added to the rcutorture/configs file.
This new directory's name can then be passed in via the kvm.sh script's
--torture parameter.

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