Test form modifies active MailSystem settings

Sorry the noise ... wrong module :)

Just got bitten by this one, I feel like config has a better way to do these sorts of temporary overrides.

Although the solution proposed in the MR is better than the current implementation it does mean that while the email is being sent any other emails sent by other requests will also use the potentially changed mailsystem settings too.
This should be isolated by using some kind of config.factory.override tagged service that'll toggle the config to what the user selected without it being saved to the persistent config storage.

In case someone else wants it, I've added a patch that simply removes the router item so that you can't get to the form and break your own site.