[mlir][vector] Update v.contract -> v.outerproduct tests (1/N) (#70379)

Tests for conversions from `vector.contract` to `vector.outerproduct`
for _matvec_ operations are updated with cases for scalable vectors.

This patch updates one specific test file (there might be similar
tests elsewhere):

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

Only the parallel dimension is made scalable. Making the reduction
dimension scalable would lead to different patterns without
`vector.outerproduct` (that would need to be added to some other file).

One duplicate test for _matvec_ is removed.