Download and install this newly packaged version of Compass for this relevant package (ie. .exe or .msi or whatever) and version of compass (regular, readonly, isolated). (See the test installer task)

Run a stubbed update server that will serve up a hardcoded binary. This hardcoded binary prints a pre-determined message to stdout. Something like "Update succeeded".

Use webdriverio to start the just-installed copy of Compass with the HADRON_AUTO_UPDATE_ENDPOINT_OVERRIDE env var, pointing it to our stubbed update server.
Make sure Compass updates by waiting for the toast to appear.

Run Compass from the command-line and check that it prints the expected message.