Enzyme and React 16 workarounds

[oandregal]

This issue is to document the workarounds that we have in place to work with some of React 16 features that aren't supported by Enzyme yet. For reference, enzymejs/enzyme#1553 is where the enzyme project is tracking their support of React 16 features.

At the time of creating this issue, we're still using enzyme 3.3.0 and enzyme-adapter-react-16 1.1.1 but the latest releases are 3.7.0 and 1.6.0. Some of the issues may have been fixed.

• Mock for forwardRef in Button.
• BlockContros's skipped test.
• Only supports the legacy context API.

[aduth]

cc @gziolo with consideration of progress in enzymejs/enzyme#1553 and enzymejs/enzyme#1604 , the current workarounds might no longer be necessary given our recent upgrade to enzyme and enzyme-adapter-react in #13922 . (There's also been another release of enzyme-adapter-react since then).

I'll poke around a bit to get a better sense of where things stand.

[gziolo]

Nice work @aduth. I wasn't sure whether we want to fix those workarounds with the updated Enzyme API or replace them with React test utils. The approach you took is probably better as it is less work :) We have over 80 test suites using enzyme so I bet we are in a position where we will have to support it for quite some time knowing that they will always be a few weeks or even months behind React releases. I hope that at some point we will build our own abstractions which will allow to replace shallow, mount and render easily. We are not there yet.