generates a field reference in the child class that points to the mixin
Inheritance: MixinCommandBase
        public void CanExecute_False()
        {
            WithSourceFiles(Files.ChildClass, Files.Mixin);
            var childClass = CreateClass(nameof(SimpleChildClass));
            var mixinClass = CreateMixinReference("_mixin", nameof(SimpleMixin));

            var command = new AddFieldDeclarationForMixinCommand(mixinClass);
            Assert.False(command.CanExecute(childClass));
        }
        public void Execute_AddFieldReference()
        {
            WithSourceFiles(Files.ChildClass, Files.Mixin);
            var childClass = CreateClass(nameof(SimpleChildClassWithoutField));
            var mixin = CreateMixinReference("_mixin", nameof(SimpleMixin));

            var command = new AddFieldDeclarationForMixinCommand(mixin);
            var newClassDeclaration = command.Execute(childClass.SourceCode, Semantic);
            var mixinField = newClassDeclaration.FindMixinReference("_mixin");

            Assert.NotNull(mixinField);
        }