Add integration tests for React 16

[alexeyr-ci1]

Since spec/dummy is now using React 18 (#1463), and we still want compatibility with React 16, we need integration tests for React 16. Could add an example to generators, but I am not sure that's enough (still better than nothing).

@justin808 What do you think? Anyone we could assign this to?

[justin808]

Ideally, we'd swap the dependency with an ENV flag so that spec/dummy can run on React 16. However, I don't see that as a priority right now.

I'll leave this one up for grabs.