Tests should run in GitHub Actions against a local server (with and without Maintenance mode)

[ghost]

Currently tests only run after deployment, against the relevant server. As this can't change whether Maintenance mode is on or not, only some of the tests run. Also, if there are any problems it's already deployed so it's to late.

It would be better if tests ran fully in GH actions, then we can test code before it's merged to main/develop. We can also test both with and without Maintenance mode.

It may be very difficult to fully set up test servers in GH actions tho, but we can look and see.

Could update readme so dev's are aware?