Allow webserver_webhook_test to be run parallel

It was working previously because of the order of tests coincidentally
configuring this plugin in an earlier-run test.
Split bot logic out of FullStackTest
Add additional FullStackTest documentation
Reorder testbackend imports
Remove unused console logger in Testbackend
Fix setting loglevel in TestBackend
Add pytest equivalent of FullStackTest
Rewrite tests/webhooks_tests.py to use pytest
Set default pytest options
Use pytest instead of nose in run_tests.py
Let testbot fixture return bot instance
More Pythonic names for popMessage, pushMessage and friends