Implement methods for kron with a Diagonal argument (#30232)
* This implements methods for kron(::Diagonal, ::AbstractMatrix) and
kron(::AbstractMatrix, ::Diagonal). These methods are typically
twice as fast as the fallback methods when the other argument
is a Matrix.
* The test for the existing method for kron(::Diagonal, ::Diagonal)
was missing the @test macro, probably inadvertently. I added the
macro and checked that the test indeed passes.
