docs: test env different from production (#7710)

Co-authored-by: Bjorn Lu <bjornlu.dev@gmail.com>

CONTRIBUTING.md

@@ -95,6 +95,8 @@ test('should work', async () => {
 
 Some common test helpers, e.g. `testDir`, `isBuild` or `editFile` are available in `packages/playground/testUtils.ts`.
 
+Note: The test build environment uses a [different default set of Vite config](https://github.com/vitejs/vite/blob/9c6501d9c363eaa3c1e7708d531fb2a92b633db6/scripts/jestPerTestSetup.ts#L102-L122) to skip transpilation during tests to make it faster. This may produce a different result compared to the default production build.
+
 ### Extending the Test Suite
 
 To add new tests, you should find a related playground to the fix or feature (or create a new one). As an example, static assets loading are tested in the [assets playground](https://github.com/vitejs/vite/tree/main/packages/playground/assets). In this Vite App, there is a test for `?raw` imports, with [a section is defined in the `index.html` for it](https://github.com/vitejs/vite/blob/71215533ac60e8ff566dc3467feabfc2c71a01e2/packages/playground/assets/index.html#L121):

scripts/jestPerTestSetup.ts

@@ -123,6 +123,7 @@ beforeAll(async () => {
           }
         },
         build: {
+          // esbuild do not minify ES lib output since that would remove pure annotations and break tree-shaking
           // skip transpilation during tests to make it faster
           target: 'esnext'
         },