add @ember/string for tests

Author: mansona

packages/ember-cli-fastboot/package.json

@@ -50,6 +50,7 @@
   },
   "devDependencies": {
     "@ember/optional-features": "^2.0.0",
+    "@ember/string": "^3.1.1",
     "@ember/test-helpers": "^2.6.0",
     "@embroider/test-setup": "^0.37.0",
     "@glimmer/component": "^1.0.4",