Add (broken) tests for allocation in 5-arg mul!()

See JuliaLang#47206 for a fix proposal.

stdlib/LinearAlgebra/test/matmul.jl

@@ -978,5 +978,18 @@ end
         @test *(a, b, c, 99.9) ≈ (a * b) * (c * 99.9)
     end
 end
+
+@testset "Issue #46865: mul!() with non-const alpha, beta" begin
+    f!(C,A,B,alphas,betas) = mul!(C, A, B, alphas[1], betas[1])
+    alphas = [1.0]
+    betas = [0.5]
+    for d in [2,3,4]  # test native small-matrix cases as well as BLAS
+        A = rand(d,d)
+        B = copy(A)
+        C = copy(A)
+        f!(C, A, B, alphas, betas)
+        @test_broken (@allocated f!(C, A, B, alphas, betas)) == 0
+    end
+end
 
 end # module TestMatmul