[mlir][linalg] Add a test for linalg.matmul --> vector.outerproduct

Representing matmuls as a sum of outer products is central to various
matrix extensions (e.g. Arm's SME). This test demonstrates how to use
Linalg's vectoriser and Vector's lowerings to represent `linalg.matmul`
as a chain of `vector.outerproduct` Ops.

Differential Revision: https://reviews.llvm.org/D150457