do not hoist @ember/test-helpers

PR emberjs/ember-test-helpers#1211 (released in v2.8.0)
made `ember-source` a peerDependency of `@ember/test-helpers`.

In case of `ember-keyboard` monorepo setup, package `ember-source`
does not get hoisted and stay in the `test-app/node_modules` but at the same time
`@ember/test-helpers` gets hoisted as well as some another version of `ember-source`.

By not hoisting `@ember/test-helpers` in the `test-app` package we avoid this issue.

test-app/package.json

@@ -84,5 +84,8 @@
   },
   "volta": {
     "extends": "../package.json"
-  }
+  },
+  "nohoist": [
+    "@ember/test-helpers"
+  ]
 }