Single<Post> save(Post post) {
        Post saved = Post.builder().id(UUID.randomUUID()).title(post.getTitle()).content(post.getContent()).build();
        DATA.add(saved);
        return Single.just(saved);
    }
@Test
    void save() {
        var testObserver = new TestObserver<Post>();
        this.posts.save(Post.builder().title("test title").content("test content").build())
                .subscribe(testObserver);

        testObserver.assertComplete();
        testObserver.assertNoErrors();
        testObserver.assertValue(p -> null != p.getId());
    }