Tests fail when SLDdb is down

[aglavic]

Several unit tests for the sample model depend on resolving materials from SLDdb, so they fail if the database can't be accessed.

It will take some effort to make this work, but we should setup the tests to generate an own instance of the SLDdb locally and use this for testing, so no access via internet is required.

[llimeht]

+1 to this. In the downstream context, Debian packages are built in environments that don't have internet access (to ensure that the archive is self-hosting and not reliant on precompiled things from outside). So far we've just worked around this with an ever-growing number of pytest.mark.skip annotations, but it would obviously be better if we were running as much of the test suite as possible as part of package build and CI.