Streamline variogram tests to reduce duplication

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maybe also set w, w_vs? Otherwise these aren't tested at all

Originally posted by @sbfnk in #1114

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and https://github.com/epiforecasts/scoringutils/pull/1115/changes/BASE..fd2608455aa6337af25499eb13eea47aea402599#r2849149848

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and #1115

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I think we should do this one alongside #1125