[MLIR][Vector] Refactor tests for contract -> OP transforms (3/N) (#73447)

This patch refactors tests for:

      vector.contract -> vector.outerproduct

for matvec operations (b += Ax). Summary of changes:
  * names of LIT variables are unified,
  * "plain" tests (i.e. without masking and with fixed-width vectors)
    are moved to the top of their respective sections,
  * missing "plain" cases 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 tests so that it's
easier to identify what cases are tested and where to add tests for
scalable vectors.

Implements #72834.