rcutorture: Print end-of-test state

This commit adds end-of-test state printout to help check whether RCU
shut down nicely.  Note that this printout only helps for flavors of
RCU that are not used much by the kernel.  In particular, for normal
RCU having a grace period in progress is expected behavior.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>