With this we can perform new assertions on the returned value without having
to cache it with an outer variable or wrapping all subsequent assertions inside
the `assert_difference` block.

Before:

```
post = nil
assert_difference -> { Post.count }, 1 do
  Post.create
end

assert_predicate post, :persisted?
```

Now:

```
post = assert_difference -> { Post.count } do
  Post.create
end

assert_predicate post, :persisted?
```