rcutorture: Vary forward-progress test interval

Some of the Linux kernel's RCU implementations provide several mechanisms
to promote forward progress that operate over different timeframes.
This commit therefore causes rcu_torture_fwd_prog() to vary the duration
of its forward-progress testing in order to test each such mechanism.

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