Write tests for both with `async` and without

[roncohen]

We need a test that exercises the command queue system both when Opbeat is loaded with async and without. This fixed the non-async case which was unfortunately broken: #33

[hmdhk]

For vanilla js: If the async attribute is used when including our script from a CDN, the snippet will queue up commands until the script is loaded: https://opbeat.com/docs/articles/get-started-with-javascript/#async