call np.cross with 3D vectors only

[keewis]

• towards ⚠️ Nightly upstream-dev CI failed ⚠️ #8844

In the tests, we've been calling np.cross with vectors of 2 or 3 dimensions, numpy>=2 will deprecate 2D vectors (plus, we're now raising on warnings). Thus, we 0-pad the inputs before generating the expected result (which generally should not change the outcome of the tests).

For a later PR: add tests to check if xr.cross works if more than a single dimension is present, and pre-compute the expected result. Also, for property-based testing: the cross-product of two vectors is perpendicular to both input vectors (use the dot product to check that), and its length (l2-norm) is the product of the lengths of the input vectors.

[dcherian]

Thanks for all your work here @keewis !