selftests/sched_ext: Add tests for SCX_ENQ_IMMED and scx_bpf_dsq_reenq()

selftests/sched_ext: Add tests for SCX_ENQ_IMMED and scx_bpf_dsq_reenq()

Add three selftests covering features introduced in v7.1:

- dsq_reenq: Verify scx_bpf_dsq_reenq() on user DSQs triggers
  ops.enqueue() with SCX_ENQ_REENQ and SCX_TASK_REENQ_KFUNC in
  p->scx.flags.

- enq_immed: Verify SCX_OPS_ALWAYS_ENQ_IMMED slow path where tasks
  dispatched to a busy CPU's local DSQ are re-enqueued through
  ops.enqueue() with SCX_TASK_REENQ_IMMED.

- consume_immed: Verify SCX_ENQ_IMMED via the consume path using
  scx_bpf_dsq_move_to_local___v2() with explicit SCX_ENQ_IMMED.

All three tests skip gracefully on kernels that predate the required
features by checking availability via __COMPAT_has_ksym() /
__COMPAT_read_enum() before loading.

Signed-off-by: zhidao su <suzhidao@xiaomi.com>
Signed-off-by: Tejun Heo <tj@kernel.org>