rcutorture: Add BUSTED-BOOST to test RCU priority boosting tests

This commit adds the BUSTED-BOOST rcutorture scenario, which can be
used to test rcutorture's ability to test RCU priority boosting.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/rcutorture/configs/rcu/BUSTED-BOOST

+CONFIG_SMP=y
+CONFIG_NR_CPUS=16
+CONFIG_PREEMPT_NONE=n
+CONFIG_PREEMPT_VOLUNTARY=n
+CONFIG_PREEMPT=y
+#CHECK#CONFIG_PREEMPT_RCU=y
+CONFIG_HZ_PERIODIC=y
+CONFIG_NO_HZ_IDLE=n
+CONFIG_NO_HZ_FULL=n
+CONFIG_RCU_TRACE=y
+CONFIG_HOTPLUG_CPU=y
+CONFIG_RCU_FANOUT=2
+CONFIG_RCU_FANOUT_LEAF=2
+CONFIG_RCU_NOCB_CPU=n
+CONFIG_DEBUG_LOCK_ALLOC=n
+CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
+CONFIG_RCU_EXPERT=y

tools/testing/selftests/rcutorture/configs/rcu/BUSTED-BOOST.boot

+rcutorture.test_boost=2
+rcutorture.stutter=0
+rcutree.gp_preinit_delay=12
+rcutree.gp_init_delay=3
+rcutree.gp_cleanup_delay=3
+rcutree.kthread_prio=2
+threadirqs
+tree.use_softirq=0