[DEV] Fix unittests (#222)

* update static unittest files macOS

- due to changes in included OSM elements from #206 b8f4d16 and #201 f6a4b99

* Adjust test_constants to match the new tags added to tags-to-keep.json

* update static unittest files Windows

- plus adding 100/137 as this was not inclued before in Windows, but in macOS
- due to changes in included OSM elements from #206 b8f4d16 and #201 f6a4b99

* integrate unittests into CONTRIBUTING.md

- incl. refactoing the file & link to land-poligons
- remove not needed headlines

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Co-authored-by: Alf Høgemark <alf@i100.no>

Author: treee111