refactor: use execSync for tests (#186)

Author: JustinBeckwith

packages/google-cloud-bigquery-datatransfer/samples/package.json

@@ -18,7 +18,6 @@
   },
   "devDependencies": {
     "chai": "^4.2.0",
-    "execa": "^1.0.0",
     "mocha": "^6.0.0"
   }
 }