public void SignOut_With_Null_Parameter_Should_Throw_ArgumentNullException()
        {
            var service          = new CollaboratorsService();
            Func <ValueTask> act = () => service.SignOut(null);

            act.Should().ThrowExactly <ArgumentNullException>();
        }
        public async Task SignOut_With_Not_Null_Parameter_Collaborators_Should_Not_Contain_It(string userName)
        {
            var service      = new CollaboratorsService();
            var identityUser = new IdentityUser {
                Id = userName, NormalizedUserName = userName
            };

            var collaborator = await service.SignIn(identityUser);

            await service.SignOut(identityUser);

            service.Collaborators.Should().NotContain(collaborator);
        }