Pusher infrastructure

Some sytests, and much of the IM functionality and experience in matrix, is dependent on pushers working correctly.

However, complement doesn't yet set up much in the way of pushers, such as dummy targets.

If these targets could be set up in a similar vein such as deployments, testing pushers could be easy.

Start by reviewing how Complement currently sets up deployments and identify the sytests and IM functionality that depend on pushers. Define the required dummy targets and verify that the resulting setup makes pusher behavior testable through the affected sytests.

Written by the indexing model from the issue text.