Add one more gradient test

Author: asterycs

test/StdTest.jl

@@ -352,6 +352,7 @@ end
     @test to_std_string(gradient(sum(x .^ 2), x)) == "2x"
     @test to_std_string(gradient(sum(x .^ 3), x)) == "3x²"
     @test to_std_string(gradient(sum(x)^2, x)) == "2sum(xᵀ)vec(1)"
+    @test to_std_string(gradient(sum(x .^ 2)^2, x)) == "2sum(xᵀ²)2x"
     @test to_std_string(gradient(sum((x + y) .^ 2), x)) == "2(x + y)"
     @test to_std_string(gradient(sum((x .* y) .^ 2), x)) == "2(x ⊙ y ⊙ y)"
     @test to_std_string(gradient(sum((A * x - y) .^ 2), x)) == "2Aᵀ(Ax - y)"