rcutorture: Add a scenario for Classic SRCU

A robust combination of paranoia and cowardice has resulted in retaining
Classic SRCU (CONFIG_CLASSIC_SRCU) as a backup for the shiny new Tiny
and Tree SRCU implementations.  If it is to be a viable backup, it of
course needs to be tested.  This commit therefore adds an rcutorture
scenario named SRCU-C for Classic SRCU.  This commit also adds this
scenario to the set that are run by default.

Once sufficient good experience has accumulated for Tiny and Tree SRCU,
this test will be removed, along with the Classic SRCU implementation
itself.

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