LinearAlgebra: Add test "matrix x matrix with negative stride". (#39849)

(Made while looking at #39836, which is matrix x vector.
Matrix x matrix with negative stride is fine as it is, because
it uses the Julia implementation of matrix multiplication.)

stdlib/LinearAlgebra/test/matmul.jl

@@ -205,6 +205,14 @@ end
     @test *(Asub, adjoint(Asub)) == *(Aref, adjoint(Aref))
 end
 
+@testset "matrix x matrix with negative stride" begin
+    M = reshape(map(Float64,1:77),7,11)
+    N = reshape(map(Float64,1:63),9,7)
+    U = view(M,7:-1:1,11:-2:1)
+    V = view(N,7:-1:2,7:-1:1)
+    @test U*V ≈ Matrix(U) * Matrix(V)
+end
+
 @testset "dot product of subarrays of vectors (floats, negative stride, issue #37767)" begin
     for T in (Float32, Float64, ComplexF32, ComplexF64)
         a = Vector{T}(3:2:7)