selftests/sched_ext: Add cyclic SCX_KICK_WAIT stress test

selftests/sched_ext: Add cyclic SCX_KICK_WAIT stress test

Add a test that creates a 3-CPU kick_wait cycle (A->B->C->A). A BPF
scheduler kicks the next CPU in the ring with SCX_KICK_WAIT on every
enqueue while userspace workers generate continuous scheduling churn via
sched_yield(). Without the preceding fix, this hangs the machine within seconds.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Tested-by: Christian Loehle <christian.loehle@arm.com>