test: address flaky test (#573)

Not really sure why the test is flaky.  Maybe without the await
statement, the process exist before the console is written to.

Fixies #565

asset/snippets/quickstart.js

@@ -49,7 +49,7 @@ async function main(assetNames) {
  console.log(util.inspect(result, {depth: null}));
  // [END asset_quickstart]
  }
- quickstart();
+ await quickstart();
 }
 
 main(...process.argv.slice(2));