add more cases in clock for mtbf cases.
add cases in clock for MTBF.

test_mtbf_clock_delete_alarm.py
test_mtbf_clock_set_alarm_repeat.py
test_mtbf_clock_set_alarm_snooze.py
Merge pull request #3 from brhuang/mtbf_test

add more cases in clock for mtbf cases.