Tweak AsyncSocketListener in tests (#1382)

* Tweak AsyncSocketListener in tests

Some CI runs have been crashing lately from within AsyncSocketListener.
This moves a bit of disconnection & exception handling around. It probably won't
fix the underlying tests which would have otherwise failed, but it might stop the
process from outright crashing.

Also reference the same code for the integration tests.

* Simplify the diff

* Stabilise a couple of tests

Author: Rob-Hague