Rename react-dom/testing to react-dom/unstable_testing in yarn build-for-devtools (#24364)

The shell package wasn't compiling because yarn build-for-devtools was incorrect. The react-dom/test package was renamed to react-dom/unstable_testing. This PR fixes this in the package.json.

Note: Adding packages to the yarn build-for-devtools command isn't great in the long run. Eventually we should make devtools have its own build script.