[List] Use stable context API

[eps1lon]

Part of #13394

Not a big fan of MergeWithListContext but ListItem would've been pretty bloated since it has two return statements.

[oliviertassinari]

@eps1lon This looks great. I'm fixing the tests, at least, trying.

[oliviertassinari]

@eps1lon If you see anything that is wrong, feel free to raise it :).

[eps1lon]

Thanks for cleaning that up! Didn't really have the time to follow through.

I had a different approach though. Will open another PR to get your feedback. I think explicitly looking at a particular child of the component is not really the intention of some of those tests. As soon as we introduce another component into the tree it will likely break again.

[oliviertassinari]

@eps1lon I ran out of ideas, if you think it can be improved, I'm all in! Also, It's likely that we will use the hook in the future to implement the feature. We need to be resilient to that change. I think that using mount over shallow is already a good first step.

[Ak-lee]

I find this error only throw out in particular case。
https://github.com/Ak-lee/webpack-server-render-debug
when I enter the directory , only when I run npm run dev:client , then in a new cmd window run npm run dev:server
open the browser at http://'localhost:3333, then the error throw out.
I run the npm run dev:client and npm run dev:server in order to achieve the server automatic从rendering while development.
if I run the npm run dev:client and open the browser at http://localhost:8888 ，there is no error throw out .
if I run the npm run build then npm start and open the browser at http://localhost:3333, both there is no error throw out.
I am sorry that I have't simplify the demo, because the error really throw out in some particular case.

[eps1lon]

@Ak-lee please open a separate. I don't understand how this is related to this PR. Maybe #13716 is related?