Also test for mutable tangent

Author: oxinabox

test/tangent_types/structural_tangent.jl

@@ -370,6 +370,11 @@ end
             a = Tangent{Tuple{Float64, Float64}}(2.0, 3.0)
             b = Tangent{Tuple{Float64, Float64}}(1.0, 1.0)
             @test (a - b) == Tangent{Tuple{Float64, Float64}}(1.0, 2.0)
+
+            a = MutableTangent{MFoo}(x=1.5, y=1.5)
+            b = MutableTangent{MFoo}(x=0.5, y=0.5)
+            @test (a - b) == MutableTangent{MFoo}(x=1.0, y=1.0)
+            
         end
 
         @testset "scaling" begin