Shallow renderer: support multiple setState invocation

[Hypnosphi]

Issue: #11161

This PR ensures that subsequent calls to setState:

1. Use the result of previous setState as base state when shallow merging
2. Pass it to updater function if any

[gaearon]

cc @bvaughn who I believe originally wrote it. Looks good to you?

[bvaughn]

It's been a while since I've thought about this component too closely 😁 but this change looks good to me. I appreciate the newly-added tests.

[gaearon]

@Hypnosphi Could you please also get a local clone of Enzyme running and verify that patching this up in react-test-utils package manually doesn't break any Enzyme unit tests?

[Hypnosphi]

I did:

# in react directory
yarn build
cd build/packages/react-test-renderer
yarn link
cd ../../../.. # parent of react directory

git clone https://github.com/airbnb/enzyme.git
cd enzyme
yarn
yarn react:16
yarn link react-test-renderer
yarn test:only

And it passes

[gaearon]

Sweet. Thanks.