Write black-box tests for DefaultAbly.connect

These document the current behaviour of this method.

I wrote them having only seen the Ably interface (which DefaultAbly
implements), and knowing the dependencies that DefaultAbly has.

I intentionally did not look at the code whilst writing these tests because I
wanted to limit myself to writing test cases based on scenarios that I could
imagine might occur, instead of ones based on logic that I know to exist.

Once I’ve written tests like this for the whole class, then I’d like to look at
its implementation and add any further test cases for logic that wasn’t clear
from the public interface.

This is part of #869.