[mlir][linalg][nfc] Clean up vectorization tests (#163379)

* Added high-level section labels in linalg-ops-with-patterns.mlir.
* Moved tests for `memref.copy` to the bottom, after all Linalg ops.
* Removed duplicate `@test_vectorize_padded_pack_no_vector_sizes` tests
  - they differed only in tensor dimensions (both static).
* Updated comments and test names for `linalg.pack` to improve clarity
  and align with https://mlir.llvm.org/getting_started/TestingGuide/.
  * Re-grouped tests for `linalg.pack`.

For a broader context, I plan to update the vectorization logic for
`linalg.pack`. This clean-up will make the following PRs easier to
review.

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