DRTVWR-476: Make LoginListener::waitFor() take arbitrary predicate.

This allows one of the tests to specifically waitFor() the completion status
update from LLLogin, rather than the next status update to come along: the
coroutine potentially emits a whole sequence of status updates before
completion.

Then the waitFor() overload that merely waits for the next status update is
implemented by passing that specific predicate to the other overload.