Fix webtests

[teutoburg]

The issue

Even though most of the tests in this package are not labelled as webtest, they still require a network connection if no cache is present (as it is the case in the CI). This causes occasional failures, see e.g. https://github.com/AstarVienna/speXtra/actions/runs/10289787099/job/28478293870

The cause

I attempted to run the tests locally after deleting the cache folder and disabling my internet connection, and it's not even possible to initialize the pytest session, because this already requires a download (unless cached). After "solving" this by running only the single webtest-marked test with a network connection to cache the initially required file(s), running the non-webtests without network connection is possible, but produces 42 errors, 27 failed and only 14 passed out of 83 tests.

Required actions

• Figure out what files are actually needed to at least initialize the pytest session
• If those are not too large, maybe keep them in the repo
• Mark all test as webtest that actually require a network connection on an empty cache
• Look into maybe using ScopeSim_Data for the CI runs to avoid all the downloading
• In the long term, try to reduce the network/cache-dependency of out tests