selftests/sched_ext: fix build after renames in sched_ext API

selftests/sched_ext: fix build after renames in sched_ext API

The selftests are falining to build on current tip of bpf-next and
sched_ext [1]. This has broken BPF CI [2] after merge from upstream.

Use appropriate function names in the selftests according to the
recent changes in the sched_ext API [3].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=fc39fb56917bb3cb53e99560ca3612a84456ada2
[2] https://github.com/kernel-patches/bpf/actions/runs/11959327258/job/33340923745
[3] https://lore.kernel.org/all/20241109194853.580310-1-tj@kernel.org/

Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Acked-by: Andrea Righi <arighi@nvidia.com>
Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Tejun Heo <tj@kernel.org>