Add Reactor.WaitForIdle

Given fuzzing is deterministic (any more data or work to handle will be posted
by existing work) we can wait for the reactor to become idle.

Add support for this and begin using in the fuzz tests.

Test: fuzz/run --host bluetooth_gd_hci_fuzz_test
Change-Id: Ib6acaf159348f886e6dc7c081a1a081df8f460bd