Tests: Workaround issues with QUnit Chrome bridge

The Chrome bridge from `grunt-contrib-qunit` is now getting injected into
every single iframe, including an empty one that has no intention of
running QUnit tests. Since that bridge requires QUnit, it fails with an
error in such cases. Workaround the issue by wrapping the bridge in
another function that bails early if QUnit is not defined.

Author: mgol

Gruntfile.js

@@ -214,7 +214,13 @@ grunt.initConfig( {
 				]
 			},
 			inject: [
-				require.resolve( "grunt-contrib-qunit/chrome/bridge" )
+				require.resolve(
+					"./tests/lib/grunt-contrib-qunit-bridges/bridge-wrapper.js.intro"
+				),
+				require.resolve( "grunt-contrib-qunit/chrome/bridge" ),
+				require.resolve(
+					"./tests/lib/grunt-contrib-qunit-bridges/bridge-wrapper.js.outro"
+				)
 			],
 			page: {
 				viewportSize: { width: 700, height: 500 }

tests/lib/grunt-contrib-qunit-bridges/bridge-wrapper.js.intro

@@ -0,0 +1,20 @@
+// The bridge in `node_modules/grunt-contrib-qunit/chrome/bridge.js` is injected
+// into every iframe, even an empty one injected during QUnit tests. The bridge,
+// in turn, requires QUnit to be present on the page, throwing errors otherwise.
+// To workaround that, add another wrapper which detects a missing QUnit and skips
+// the whole logic.
+
+( function ( factory ) {
+	if ( typeof define === 'function' && define.amd ) {
+		require( [ 'qunit' ], factory );
+	} else {
+		factory( window.QUnit );
+	}
+} )( function( QUnit ) {
+
+if ( !QUnit ) {
+
+	// No QUnit => possibly an empty iframe injected in tests; ignore.
+	return;
+}
+

tests/lib/grunt-contrib-qunit-bridges/bridge-wrapper.js.outro

@@ -0,0 +1,2 @@
+
+} );