selftests/timens: handle a case when alarm clocks are not supported

This can happen if a testing node doesn't have RTC (real time clock)
hardware or it doesn't support alarms.

Fixes: 61c57676

 ("selftests/timens: Add Time Namespace test for supported clocks")
Acked-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reported-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>