rcutorture: Add mid-sized stall to TREE07

There is code in rcu_implicit_dynticks_qs() that checks for the current
grace period being halfway to the RCU CPU stall timeout, but rcutorture
currently does not test this code.  This commit therefore adds a 14-second
stall to the TREE07 scenario in order to test this code given the default
RCU CPU stall warning timeout of 21 seconds.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>

tools/testing/selftests/rcutorture/configs/rcu/TREE07.boot

 nohz_full=2-9
+rcutorture.stall_cpu=14
+rcutorture.stall_cpu_holdoff=90
+rcutorture.fwd_progress=0