add test

Author: Miha Zgubic

test/tangent_types/tangent.jl

@@ -324,6 +324,7 @@ end
     @testset "-Tangent" begin
         t = Tangent{Foo}(; x=1.0, y=-2.0)
         @test -t == Tangent{Foo}(; x=-1.0, y=2.0)
+        @test -1.0*t == -t
     end
 
     @testset "scaling" begin