Fixed appveyor again, no more eager tests.

Forgot to convert bash commands to batch commands in appveyor.yml.
Setting env variables properly.

Instead of using CELERY_ALWAYS_EAGER, I'm launching a real Celery
working in a thread under the same Python process.

Moved Celery and Flask app instances into a dedicated module. No longer
testing on different Flask/Celery instances, just one instance of each.