alarm_unittest: Add TwoAlarmTest

This test schedules a new alarm from the context of the first
alarm.  It uses fake_timer_fd, and advances the timer enough
so that the events are always ready.

Bug: 331972155
Test: atest bluetooth_test_gd_unit
Flag: EXEMPT, testing-only change
Change-Id: I652b2887303061a5177d211aac1136040d2975ef