Add gradient test

Author: asterycs

test/StdTest.jl

@@ -285,6 +285,7 @@ end
     @test to_std_string(gradient(sum(x .^ 3), x)) == "3x²"
     @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)"
 end
 
 @testset "to_std_string of jacobian {A, A'} * x" begin