Add cases in clock for MTBF.

new file:   test_mtbf_clock_set_alarm_sound.py
new file:   test_mtbf_clock_set_alarm_time.py
new file:   test_mtbf_clock_switch_clock_type.py
Merge pull request #4 from brhuang/mtbf_1203

Add cases in clock for MTBF.