update no-unknown-modifier rules, resolve #107 (#109)

Author: cgcgbcbc

rules/no-unknown-modifiers.js

@@ -11,7 +11,8 @@ var modifiers = [
 	'only',
 	'serial',
 	'skip',
-	'todo'
+	'todo',
+	'failing'
 ];
 
 function getTestModifiers(node) {

test/no-unknown-modifiers.js

@@ -27,6 +27,7 @@ test(() => {
 			`${header} test.todo(t => {});`,
 			`${header} test.after.always(t => {});`,
 			`${header} test.afterEach.always(t => {});`,
+			`${header} test.failing(t => {});`,
 			// shouldn't be triggered since it's not a test file
 			`test.foo(t => {});`
 		],