Add watcher to test

test/cli/samples/watch/watch-config-early-update/_config.js

@@ -9,24 +9,29 @@ module.exports = {
 	description: 'immediately reloads the config file if a change happens while it is parsed',
 	command: 'rollup -cw',
 	before() {
+		// This test writes a config file that watches itself for changes and resolves only once it has
+		// been rewritten. As soon as that watcher is set up, the config file writes a message to stderr
+		// that is observed by the parent process and triggers overwriting the config file.
+		// That way, we reliably simulate a complicated config file being changed while it is parsed.
 		fs.mkdirSync(path.resolve(__dirname, '_actual'));
 		configFile = path.resolve(__dirname, 'rollup.config.js');
 		fs.writeFileSync(
 			configFile,
 			`
-			console.error('initial');
+		  import chokidar from 'chokidar';
       export default new Promise(resolve => {
-        setTimeout(
-          () =>
-            resolve({
+				const watcher = chokidar.watch(${JSON.stringify(configFile)})
+				  .on('change', () => {
+				    watcher.close();
+				    setTimeout(() => resolve({
               input: { output1: 'main.js' },
               output: {
                 dir: '_actual',
                 format: 'es'
               }
-            }),
-          1000
-        );
+            }), 300)
+				  });
+				console.error('initial');
       });
   		`
 		);