Closes #2400 Unified input date object for `derive_vars_dt()` test suite

[zdz2101]

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[StefanThoma]

I had a look and it looks really good! I think the suppression of the warnings is fine in this context. The warnings themselves should be checked in a separate test IMO.

[zdz2101]

@bms63 @StefanThoma @millerg23 Added in a separate input object that plugs in for its appropriate warnings but also checks the return object is still valid