Add more gradient tests

Author: asterycs

test/StdTest.jl

@@ -345,6 +345,10 @@ end
     @test to_std_string(gradient(sum(x), x)) == "vec(1)"
     @test to_std_string(gradient(2 * sum(x), x)) == "2vec(1)"
     @test to_std_string(gradient(sum(2 * x), x)) == "2vec(1)"
+    @test to_std_string(gradient(2 * sum(sin(x)), x)) == "2cos(x)"
+    @test to_std_string(gradient(sum(2 * sin(x)), x)) == "2cos(x)"
+    @test to_std_string(gradient(2 * sum(cos(A * x + y)), x)) == "2(-1)Aᵀsin(Ax + y)"
+    @test to_std_string(gradient(sum(2 * cos(A * x + y)), x)) == "2(-1)Aᵀsin(Ax + y)"
     @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 + y) .^ 2), x)) == "2(x + y)"