Add unit test for Vue.mixin

Author: armano2

tests/lib/utils/vue-component.js

@@ -120,6 +120,12 @@ function invalidTests (ext) {
       parserOptions,
       errors: [makeError(1)]
     },
+    {
+      filename: `test.${ext}`,
+      code: `Vue.mixin({})`,
+      parserOptions,
+      errors: [makeError(1)]
+    },
     {
       filename: `test.${ext}`,
       code: `