Improve filter test coverage

[aaron-hopkinson]

Some of the filters have very little/no test coverage. We should implement tests to verify the correct behaviour and prevent regressions.

The following filters have been identified as having no/little test coverage:

• icon_refinement_level
• land_parameters
• oras6_clipping
• repeat_members
• snow_cover
• timeseries
• w_to_wz

NB: regrid could also do with more thorough tests (e.g. asserting the result is correct).