[mlir][vector] Update tests for collapse 2/n (nfc) (#94604)

The main goal of this PR (and subsequent PRs), is to add more tests with
scalable vectors to:
  * vector-transfer-collapse-inner-most-dims.mlir

Changes in this PR:

1. Renamed `@contiguous_inner_most_dim_bounds` as
   `@contiguous_inner_most_dim_with_subview`. This test was introduced
   to make sure that the `in_bounds` attribute is correctly preserved,
   but that's already verified by some earlier tests. The updated name
   highlights the differentiating factor of this test when compared to
   the other tests _currently_ present in the file, i.e. the presence of
   `memref.subview` in the input IR.

2. Renamed `@contiguous_inner_most_dim_out_of_bounds_2d` as
   `@negative_non_unit_inner_vec_dim`. While this test does contain an
   out-of-bounds access, the actual reason for the tested pattern to
   fail is the fact that the inner dim in the output vector is not "1".
   A complimentary test was added to verify that the pattern also fails
   when the source memref has non-unit trailing dim
   (`@negative_non_unit_inner_memref_dim`).

3. Renamed `@contiguous_inner_most_dim` as
   `@contiguous_inner_most_dim_non_zero_idxs` - this test verifies that
   the pattern works in the presence of non-zero idxs.

4. Added more tests for scalable vectors - this should cover all cases
   for `vector.transfer_read`.

NOTE: This PR is limited to tests for `vector.transfer_read`.

Follow-up for: #94490