dillon1000/react

Change `yarn test` to default to experimental (#26741)

The idea is that the default `yarn test` command should be the one that
includes the most bleeding edge features, because during development you
probably want as many features enabled as possible.

That used to be `www-modern` but nowadays it's `experimental` because
we've landed a bunch of async actions stuff in experimental but it isn't
yet being tested at Meta.

So this switches the default to `experimental`.