kselftests: timers: Make set-timer-lat fail more gracefully for !CAP_WAKE_ALARM

The set-timer-lat test fails when testing CLOCK_BOOTTIME_ALARM
or CLOCK_REALTIME_ALARM when the user isn't running as root or
with CAP_WAKE_ALARM.

So this patch improves the error checking so we report the
issue more clearly and continue rather then reporting a failure.

Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>