test: fix appveyor

Author: Guillaume Chau

appveyor.yml

@@ -10,7 +10,7 @@ test_script:
   - node --version
   - yarn --version
   - yarn test
-  - yarn clean && cd packages/@vue/cli-ui && yarn test
+  - cd "packages/@vue/cli-ui" && yarn test
 
 cache:
   - node_modules -> yarn.lock