Fix test debugging in VS Code.

Without these environment variables, choosing "Run -> Start Debugging"
on a test file in VS Code would run into the assertion added by #5167:

```
Jest must be run from `yarn test`
```

.vscode/launch.json

@@ -5,6 +5,8 @@
     {
       "name": "Run Jest",
       "type": "node",
+      "runtimeExecutable": "yarn",
+      "runtimeArgs": ["test"],
       "program": "${workspaceFolder}/node_modules/jest/bin/jest",
       "args": ["--runInBand", "${file}"],
       "cwd": "${workspaceFolder}",