Tests: Allow running async/await in test files

We have previously not been able to use `async/await` in our test files.
This can lead to particularly deep nesting or callback-passing when we
are trying to sequence events for the purpose of the test.

In this change we're adding the `babel-polyfill` to allow us to use
those constructs and _linearize_ the async flows.

Author: dmsnell

package-lock.json

@@ -13,14 +13,15 @@
   "scripts": {
     "flow": "flow",
     "test:coverage": "nyc npm test",
-    "test": "mocha --reporter dot --require @babel/register test/**",
+    "test": "mocha --reporter dot --require @babel/register --require babel-polyfill test/**",
     "babel": "babel -q -d lib/ src/",
     "lint": "eslint --quiet src/",
     "prepare": "npm run babel"
   },
   "author": "",
   "license": "BSD-2-Clause",
   "dependencies": {
+    "babel-polyfill": "^6.26.0",
     "uuid": "3.3.3",
     "websocket": "1.0.30"
   },