Tests: Make Puppeter really fire Chrome on macOS

Also, 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.

Ref gh-2157

Gruntfile.js

@@ -209,15 +209,18 @@ grunt.initConfig( {
 		} ),
 		options: {
 			puppeteer: {
-				ignoreDefaultArgs: true,
 				args: [
-					"--headless",
-					"--disable-web-security",
 					"--allow-file-access-from-files"
 				]
 			},
 			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 @@
+
+} );