selftests/sched_ext: Update test enq_select_cpu_fails

selftests/sched_ext: Update test enq_select_cpu_fails

With commit 08699d20467b6 ("sched_ext: idle: Consolidate default idle
CPU selection kfuncs") allowing scx_bpf_select_cpu_dfl() to be invoked
from multiple contexts, update the test to validate that the kfunc
behaves correctly when used from ops.enqueue() and via BPF test_run.

Additionally, rename the test to enq_select_cpu, dropping "fails" from
the name, as the logic has now been inverted.

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