Support the default modifier

See avajs/ava#2836.

Author: novemberborn

rules/no-unknown-modifiers.js

@@ -9,6 +9,7 @@ const modifiers = new Set([
 	'always',
 	'before',
 	'beforeEach',
+	'default',
 	'only',
 	'serial',
 	'skip',

test/no-unknown-modifiers.js

@@ -17,6 +17,8 @@ ruleTester.run('no-unknown-modifiers', rule, {
 		`${header}test.afterEach(t => {});`,
 		`${header}test.before(t => {});`,
 		`${header}test.beforeEach(t => {});`,
+		`${header}test.default(t => {});`,
+		`${header}test.default.serial(t => {});`,
 		`${header}test.only(t => {});`,
 		`${header}test.serial(t => {});`,
 		`${header}test.skip(t => {});`,
@@ -83,15 +85,6 @@ ruleTester.run('no-unknown-modifiers', rule, {
 				column: 6
 			}]
 		},
-		{
-			code: `${header}test.default(t => {});`,
-			errors: [{
-				message: 'Unknown test modifier `.default`.',
-				type: 'Identifier',
-				line: 2,
-				column: 6
-			}]
-		},
 		{
 			code: `${header}test.test(t => {});`,
 			errors: [{