Add a test user to all sites

[Daniel-WWU-IT]

For health monitoring of all mesh sites, we need a special user account that is present on every site. This PR adds such a user to each users-*.json file so that every site will have the same test user credentials (which are, of course, test/testpass).

This omnipresent test user allows us to perform remote checks that require authentication using the same credentials for every site, making automated check configuration much easier.

Note that this is just a temporary solution, but for this initial phase, this is the easiest solution.

(I recreated this PR as the previous one was a mess)

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.