[MLIR][Vector] Refactor tests for contract -> OP transforms (#73348)

This patch refactors tests for:

   * vector.contract -> vector.outerproduct

transformations for matvec operations (b += Ax). Specifically, relevant
tests from the following 2 files:

   * vector-contract-matvec-transforms.mlir
   * vector-contract-to-outerproduct-transforms.mlir

are combined into one:

  * vector-contract-to-outerproduct-matvec-transforms.mlir

All original tests are preserved and no new tests are added.

This is a part of a larger effort to add cases with scalable vectors
to tests for the Vector dialect. I am refactoring these test as a
preparation for follow-up patches.

Implements #72834.