kselftests: timers: set-timer-lat: Add one-shot timer test cases

These testcases are motivated by a recent alarmtimer regression, which
caused one-shot CLOCK_{BOOTTIME,REALTIME}_ALARM timers to become
periodic timers.

The new testcases are very similar to the existing testcases for
repeating timers.  But rather than waiting for 5 alarms, they wait for 5
seconds and verify that the alarm fired exactly once.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>