docs: clarify required build before test (#7389)

Author: j-maas

CONTRIBUTING.md

@@ -10,7 +10,9 @@ To develop and test the core `vite` package:
 
 1. Run `pnpm i` in Vite's root folder
 
-2. Go to `packages/vite` and run `pnpm run dev`. This starts `rollup` in watch mode.
+2. Run `pnpm run build` in Vite's root folder.
+
+3. If you are developing Vite itself, you can go to `packages/vite` and run `pnpm run dev` to automatically rebuild Vite whenever you change its code.
 
 You can alternatively use [Vite.js Docker Dev](https://github.com/nystudio107/vitejs-docker-dev) for a containerized Docker setup for Vite.js development.
 
@@ -67,7 +69,7 @@ And re-run `pnpm install` to link the package.
 
 Each package under `packages/playground/` contains a `__tests__` directory. The tests are run using [Jest](https://jestjs.io/) + [Playwright](https://playwright.dev/) with custom integrations to make writing tests simple. The detailed setup is inside `jest.config.js` and `scripts/jest*` files.
 
-Each test can be run under either dev server mode or build mode.
+Each test can be run under either dev server mode or build mode. Make sure that [Vite has been built](#repo-setup).
 
 - `pnpm test` by default runs every test in both serve and build mode.