Wait for doc to be indexed before searching in tests

This was causing the tests on travis to intermittently fail, after a bit
of digging it turned out that I wasn't waiting for the indexing to
complete before trying to search for the document (because the POST
endpoint indexes the documents async outside the request/response cycle.

So instead of using the POST API in the tests, we simply create the doc
with mongoose and wait for its 'es-indexed' event to fire before
continuing.