[mlir][vector][nfc] Refactor vector.contract matvec tests (#72832)

Update tests in "vector-contract-matvec-transforms.mlir" so that they
are consistent with similar tests in:
 * "vector-contract-to-outerproduct-transforms.mlir".

This is to enable further refactoring in a follow-up patch, namely to:
  * remove duplication (this will be much easier once consistent naming
    is used),
  * extend tests in "vector-contract-matvec-transforms.mlir" with cases
    for scalable vectors,
  * merge "vector-contract-matvec-transforms.mlir" and
    "vector-contract-to-outerproduct-transforms.mlir"  (there's no need
    for 2 different files testing identical transformations).

Overview of changes in this patch:
  1. Simplify the test by removing MemRef wrappers - this test verifies
     Vector -> Vector transformations and  MemRefs are not needed.
  2. Use (m, k) indices instead of (i, j).
  3. Rename function names.

This is part of a larger effort to improve test coverage for scalable
vectors in the Vector dialect. Implements #72834.