fix: use latest ECMAScript version in tests (#364)

Author: aladdin-add

Diff for: tests/lib/rules/no-only-tests.js

@@ -11,7 +11,7 @@ const RuleTester = require('eslint').RuleTester;
 // Tests
 // ------------------------------------------------------------------------------
 
-const ruleTester = new RuleTester({ parserOptions: { ecmaVersion: 6 } });
+const ruleTester = new RuleTester({ parserOptions: { ecmaVersion: 'latest' } });
 ruleTester.run('no-only-tests', rule, {
   valid: [
     // No test cases with `only`

Diff for: tests/lib/rules/prefer-output-null.js

@@ -18,7 +18,7 @@ const ERROR = { messageId: 'useOutputNull', type: 'Property' };
 // Tests
 // ------------------------------------------------------------------------------
 
-const ruleTester = new RuleTester();
+const ruleTester = new RuleTester({ parserOptions: { ecmaVersion: 'latest' } });
 ruleTester.run('prefer-output-null', rule, {
   valid: [
     `