selftests/bpf: Skip timer cases when bpf_timer is not supported

selftests/bpf: Skip timer cases when bpf_timer is not supported

When enable CONFIG_PREEMPT_RT, verifier will reject bpf_timer with
returning -EOPNOTSUPP.

Therefore, skip test cases when errno is EOPNOTSUPP.

cd tools/testing/selftests/bpf
./test_progs -t timer
125     free_timer:SKIP
456     timer:SKIP
457/1   timer_crash/array:SKIP
457/2   timer_crash/hash:SKIP
457     timer_crash:SKIP
458     timer_lockup:SKIP
459     timer_mim:SKIP
Summary: 5/0 PASSED, 6 SKIPPED, 0 FAILED

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20250910125740.52172-3-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>