Add a method to run the handler's messages from the tests

This is really brutal, but after trying many many things I didn't
find a better way. It works with the current implementation of
MessageQueue.
Since this only introduces a utility method it doesn't really do
anything, but the functionality it provides will be essential to
Bug: 5975557

Change-Id: I81c4113a08f9a8d8a88294d7dd3b6c8c483c8b1d