[mlir][vector][nfc] Move tests for scalable outer-product (#76035)

Tests for vector.outerproduct for scalable vectors from
"vector-scalable-outerproduct.mlir" are moved to:

  * ops.mlir and invalid.mlir.

These files are effectively used to document what Ops are supported and
That's basically what the original file was testing (but specifically
for scalable vectors).