easier way to expect warnings in tests

test/function/confused-default-identifier/_config.js

@@ -1,5 +1,10 @@
+var assert = require( 'assert' );
+
 module.exports = {
-	description: 'Rollup should not get confused and allow "default" as an identifier name'
+	description: 'Rollup should not get confused and allow "default" as an identifier name',
+	warnings: function ( warnings ) {
+		// ignore pending gh-587
+	}
 };
 
 // https://github.com/rollup/rollup/issues/215

test/function/custom-path-resolver-async/_config.js

@@ -21,6 +21,11 @@ module.exports = {
 			}
 		}]
 	},
+	warnings: function ( warnings ) {
+		assert.deepEqual( warnings, [
+			"Treating 'path' as external dependency"
+		]);
+	},
 	exports: function ( exports ) {
 		assert.strictEqual( exports.path, require( 'path' ) );
 	}

test/function/custom-path-resolver-sync/_config.js

@@ -13,6 +13,11 @@ module.exports = {
 			}
 		}]
 	},
+	warnings: function ( warnings ) {
+		assert.deepEqual( warnings, [
+			"Treating 'path' as external dependency"
+		]);
+	},
 	exports: function ( exports ) {
 		assert.strictEqual( exports.path, require( 'path' ) );
 	}

test/test.js

@@ -133,8 +133,12 @@ describe( 'rollup', function () {
 
 			var config = loadConfig( FUNCTION + '/' + dir + '/_config.js' );
 			( config.skip ? it.skip : config.solo ? it.only : it )( dir, function () {
+				var warnings = [];
+				var captureWarning = msg => warnings.push( msg );
+
 				var options = extend( {
-					entry: FUNCTION + '/' + dir + '/main.js'
+					entry: FUNCTION + '/' + dir + '/main.js',
+					onwarn: captureWarning
 				}, config.options );
 
 				if ( config.solo ) console.group( dir );
@@ -215,6 +219,12 @@ describe( 'rollup', function () {
 							}
 						}
 
+						if ( config.warnings ) {
+							config.warnings( warnings );
+						} else if ( warnings.length ) {
+							throw new Error( `Got unexpected warnings:\n${warnings.join('\n')}` );
+						}
+
 						if ( config.show || unintendedError ) {
 							console.log( code + '\n\n\n' );
 						}