rcutorture: Add a scenario for Tiny SRCU

This commit adds an SRCU-t rcutorture scenario for the new Tiny SRCU
implementation, removing the need to pass the --bootargs parameter to
kvm.sh to run Tiny SRCU tests.  This commit also adds SRCU-t to the set
of scenarios that are run by default.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>