[mlir][Vector] Update v.contract -> v.outerproduct tests (NFC) (#70449)

Re-orders tests in vector-contract-to-outerproduct-transforms.mlir so
that the file starts as follows:

  1. plain matmul
  2. plain matmul with scalable vectors
  3. masked matmul
  4. masked matmul with scalable vectors
  5. plain matmul with mixed types
  6. plain matmul with mixed types and scalable vectors

All of the above share the same indexing maps. This allowed to identify
one more duplicate. Following the cases above are examples with
different maps.

In addition, added extra comments to document the tests and to split
them into categories. There is also some extra reformatting to unify the
tests.

Author: banach-space