selftests/sched_ext: Verify nohz_full tick behavior

selftests/sched_ext: Verify nohz_full tick behavior

Finite-slice EXT tasks need the periodic scheduler tick to expire their
slices even when nohz_full is enabled.

Add a regression test that selects a nohz_full CPU and exercises both
infinite-to-finite and finite-to-finite slice transitions across an idle
interval. For each finite task, verify that its ops.tick() callback is
invoked.

Skip the test when an allowed nohz_full CPU and a separate housekeeping
CPU are not available.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>