CI: Test mutli-year support

[euronion]

The CI currently does not test for multi-year modelling support.
After a discussion with @amos-schledorn this seems like a prudent thing to test for, given that more features in this direction are planned.

Any reason this is currently not done? @fneum @lkstrp

[fneum]

Yes, but we can’t really test more configs because the CI already takes 45+ minutes.

[lkstrp]

We have soonish the possibility to test any configs you might want to test, also longer running ones. Not via GitHub runners and we probably don't wanna trigger it per commit on PRs, but nightly or weekly on the master branch. So we can add it to the list

[amos-schledorn]

Couldn't we just move the snapshot window of the test configs to the last half week of 2012 and first half week of 2013?