Better functional test coverage for regridding

# ✨ Feature Request
In core ANTS, we should extend the rose stem tests to include all supported regridding schemes for the general regrid:

- Linear,
- TwoStage,
- ConservativeESMF,
- AreaWeighted,
- Nearest

Note that the ConservativeESMF tests should include combinations of rectilinear and curvilinear source and target.

This is a stepping stone towards work to integrate iris-esmf-regrid into regular ANTS, so the framework should be extensible to additional schemes.

Start by locating the core ANTS rose stem tests and run the existing functional test suite to understand its current structure. Extend coverage for Linear, TwoStage, ConservativeESMF, AreaWeighted, and Nearest, including rectilinear and curvilinear source and target combinations for ConservativeESMF. Done means all listed schemes are covered and the test framework can accommodate additional schemes.

Written by the indexing model from the issue text.