selftests/sched_ext: Add test for scx_bpf_select_cpu_and() via test_run

Update the allowed_cpus selftest to include a check to validate the
behavior of scx_bpf_select_cpu_and() when invoked via a BPF test_run
call.

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