Missing form data in request
You seem to be confused as to what the expected format for the post body should be. Should it be JSON data (which is what you send in the test case), or should it be in the application/x-www-form-urlencoded format (which is what you claim to send in the test case, and what the endpoint will read)?
If you wish to receive JSON data, you'll need to change the endpoint to read the data from request.get_json(). You'll also need to use application/json as the Content-Type header in the test case.
If you wish to receive urlencoded post data, then just simplify the test case by removing the Content-Type header and the json.dumps. Just pass the data dict to the data argument.