fix: add missing package (vuejs#2466)

Author: shortdiv

packages/@vue/cli-test-utils/package.json

@@ -22,6 +22,7 @@
   },
   "dependencies": {
     "execa": "^1.0.0",
+    "fs-extra": "^7.0.1",
     "json-server": "^0.14.0",
     "puppeteer": "^1.0.0",
     "strip-ansi": "^5.0.0"