Add test for checking that append! takes elements from second arg

Author: synthfi

test/test_mutable_list.jl

@@ -97,7 +97,8 @@
                 @testset "append" begin
                     l2 = MutableLinkedList{Int}(n+1:2n...)
                     append!(l, l2)
-                    @test l == MutableLinkedList{Int}(1:2n...)
+                    @test l  == MutableLinkedList{Int}(1:2n...)
+                    @test l2 == MutableLinkedList{Int}()
                     @test collect(l) == collect(MutableLinkedList{Int}(1:2n...))
                     l3 = MutableLinkedList{Int}(1:n...)
                     append!(l3, n+1:2n...)