selftests/sched_ext: Skip rt_stall on older kernels and list skipped tests

selftests/sched_ext: Skip rt_stall on older kernels and list skipped tests

rt_stall tests the ext DL server which was introduced in commit
cd959a356205 ("sched_ext: Add a DL server for sched_ext tasks").
On older kernels that lack this feature, the test calls ksft_exit_fail()
internally which terminates the entire runner process, preventing
subsequent tests from running.

Add a guard in setup() that checks for the ext_server field in struct rq
via __COMPAT_struct_has_field() and returns SCX_TEST_SKIP if not present.

Also print the names of skipped tests in the results summary, mirroring
the existing behavior for failed tests.

Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>