Fix ide typescript compatability in __tests__ dir

[tonyraoul]

Description

While having a look at #3590 I found out that WeakRef was not defined although ESNext was being in use.
Apparently VSCode and most LSP servers don't recognise tsconfig.test.json and the directory was explicitly excluded from the project root, only tsconfig.json is recognised.

This PR aims at fixing Developer Experience while working within __tests__ for both mobx-react and mobx-react-light packages.

Hope I am helping.
Cheers.

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[urugator]

Thanks for looking into it. Isn't there the same issue with mobx package?
I wasn't able to get rid of those "decorators are experimental" warnings in tests , so this could somehow explain it... (but IIRC i did tried to remove __test__ from exludes) ...

[tonyraoul]

@urugator

Isn't there the same in issue with mobx package?

Added a fix for it in 9409d73

I wasn't able to get rid of those "decorators are experimental" warnings in tests , so this could somehow explain it... (but IIRC i did tried to remove test from exludes) ...

I was able to see this issue within VSCode and after 9409d73 it's fixed.
Removing __tests__ from exclude would have solved the issue if ES2021.WeakRef was added to lib, however within the mobx-react directory lib is set to es6 which doesn't have WeakRef.

[tonyraoul]

@urugator would you like me to close this or is it still needed ?

[urugator]

No please keep it open. The PR is very much appreciated, I (or someone else) will get to it eventually. There are just other issues I would like to settle first and I don't want to accidentaly complicate their resolution due to config changes or something. Sorry for the delay.