Add test for helpful error

test/cli/index.js

@@ -13,7 +13,7 @@ const cwd = process.cwd();
 sander.rimrafSync(__dirname, 'node_modules');
 sander.copydirSync(__dirname, 'node_modules_rename_me').to(__dirname, 'node_modules');
 
-runTestSuiteWithSamples.only(
+runTestSuiteWithSamples(
 	'cli',
 	path.resolve(__dirname, 'samples'),
 	(dir, config) => {

test/cli/samples/config-no-module/_config.js

@@ -1,4 +1,16 @@
+const { assertStderrIncludes } = require('../../../utils.js');
+
 module.exports = {
 	description: 'provides a helpful error message if a transpiled config is interpreted as "module"',
-	command: 'cd sub && rollup -c'
+	command: 'cd sub && rollup -c',
+	error: () => true,
+	stderr: stderr =>
+		assertStderrIncludes(
+			stderr,
+			'[!] While loading the Rollup configuration from "rollup.config.js", Node tried to require an ES module from a CommonJS ' +
+				'file, which is not supported. A common cause is if there is a package.json file with "type": "module" in the same folder. You can ' +
+				'try to fix this by changing the extension of your configuration file to ".cjs" or ".mjs" depending on the content, which will ' +
+				'prevent Rollup from trying to preprocess the file but rather hand it to Node directly.\n' +
+				'https://rollupjs.org/guide/en/#using-untranspiled-config-files'
+		)
 };