[mlir][vector] Update tests for collapse 3/n (nfc) (#94906)

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

There's quite a few cases to consider, hence this is split into multiple
PRs. In this PR, the very first test for `vector.transfer_write` is
complemented with all the possible combinations:
  * scalable (rather than fixed) unit trailing dim,
  * dynamic (rather than static) trailing dim in the source memref.

To this end, the following tests:
  * `@leading_scalable_dimension_transfer_write`
    `@trailing_scalable_one_dim_transfer_write`

are replaced with:
  * `@drop_two_inner_most_dim_scalable_inner_dim` and
    `@negative_scalable_unit_dim`,

respectively. In addition:
  * "_for_transfer_write" is removed from function names (to reduce
    noise).

In addition, to maintain consistency between the tests for `xfer_read`
and `xfer_write`, 2 negative tests for `xfer_read` are also renamed.
This is to follow the suggestion made during the review of this PR.

Extra comments in "VectorTransforms.cpp" are added to better
document the limitations related to scalable vectors and which tests
added here excercise.

This is a follow-up for: #94490 and #94604

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

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