[mlir][vector] Update tests for xfer permutation lowering (2/N) (#123237)
Unifies test function names so that it's easier to identify what
different cases are. Also improves consistency. The following naming
scheme has been adopted:
* `@xfer_{read|write}_{map_type}_{masked|with_mask|}_{out_of_bounds}_{scalable}`
Also updated some comments to better document the patterns that are
being exercised.