Code review comment for lp:~vanvugt/mir/fix-MirClientSurfaceTest-fragility

Hmm, needing to duplicating code from MirConnection::stream_created() doesn't seem the most change-proof way of writing a *unit* test. I think the abstractions or the test (or, more likely, both) are wrong.

But this MP doesn't purport to fix the mess, just to make it work better. So OK.