Testing: Test user for services requiring authentication

While it would probably need slightly different approaches for the different archives, I think it would be extremely beneficial to figure out ways to test services that require authentication. At least we should come up with a protocol that enables a few maintainers to be able to run the tests that are otherwise defaulted to be skipped due to the missing authentication.

The possible solutions as I see (please suggest additional ones):
- set up test user that has limited access to the archive data holding, its defails then can be shipped in the library, enabling anyone to run all the tests (but would limit access to data). This potentially needs the most support from the archive sides, and not much effort from the maintainers'.
- use secrets stored in the repo either as repo secrets, or deploy key, or webhook, or etc. This info would be visible only to admins to the repo, and integrated services, e.g. github action runs that run the CI. This may or may not require some work from the archives, and some from the maintainers' side.
- requiring maintainers to set up personal user accounts and run the remote tests regularly on their computers to check on the remote tests. This would require the most non-automatable work, almost only from the maintainers' side.

Start by inventorying the archive-specific tests currently skipped because authentication is missing, then compare the three proposed approaches: limited test users, repository secrets or deploy keys, and maintainer-run accounts. Done means a documented protocol that lets selected maintainers run the authenticated tests safely, with a clear choice of how credentials are managed.

Written by the indexing model from the issue text.