Rename react-dom/testing to react-dom/unstable_testing in yarn build-for-devtools (facebook#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.

Author: lunaruan

package.json

@@ -108,7 +108,7 @@
   "scripts": {
     "build": "node ./scripts/rollup/build.js",
     "build-combined": "node ./scripts/rollup/build-all-release-channels.js",
-    "build-for-devtools": "cross-env RELEASE_CHANNEL=experimental yarn build react/index,react/jsx,react-dom/index,react-dom/test,react-is,react-debug-tools,scheduler,react-test-renderer,react-refresh,react-art --type=NODE && cp -r ./build/node_modules build/oss-experimental/",
+    "build-for-devtools": "cross-env RELEASE_CHANNEL=experimental yarn build react/index,react/jsx,react-dom/index,react-dom/unstable_testing,react-is,react-debug-tools,scheduler,react-test-renderer,react-refresh,react-art --type=NODE && cp -r ./build/node_modules build/oss-experimental/",
     "build-for-devtools-dev": "yarn build-for-devtools --type=NODE_DEV",
     "build-for-devtools-prod": "yarn build-for-devtools --type=NODE_PROD",
     "linc": "node ./scripts/tasks/linc.js",