selftests/rseq: Skip tests if time slice extensions are not available

selftests/rseq: Skip tests if time slice extensions are not available

Don't fail, skip the test if the extensions are not enabled at compile or
runtime.

Fixes: 830969e7821a ("selftests/rseq: Implement time slice extension test")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://patch.msgid.link/20260428224427.597838491%40kernel.org
Cc: stable@vger.kernel.org