fix(tslint.json): linting of test(s) folder (#924)

tests were never tslinted becauve of a typo

Author: elevatebart

packages/@vue/cli-plugin-typescript/lib/tslint.js

@@ -75,7 +75,7 @@ module.exports = function lint (args = {}, api, silent) {
 
   const files = args._ && args._.length
     ? args._
-    : ['src/**/*.ts', 'src/**/*.vue', 'test/**/*.ts']
+    : ['src/**/*.ts', 'src/**/*.vue', 'tests/**/*.ts']
 
   const stripTsExtension = str => str.replace(/\.vue\.ts\b/g, '.vue')