tests: don't use bt_os_callouts for wakelocks

Introduces alarm_set_wake_lock_paths so wake lock paths can be changed
for testing, and adds AlarmTestHarness::WakeLockHeld to test whether a
wake lock is currently held.

Bug: 25387683
Change-Id: I9a41ae8266e252a3d436f8d41ea3f9e7ecb45cdc