text: Title: How to make TestBot use text backend? (6.1)
  Body: In order to let us help you better, please fill out the following fields as best you can:
  
  ### I am...
  
  * [ ] Reporting a bug
  * [ ] Suggesting a new feature
  * [ ] Requesting help with running my bot
  * [X] Requesting help writing plugins
  * [ ] Here about something else
  
  ### I am running...
  
  * Errbot version: 6.1.2
  * OS version: Windows
  * Python version: 3.6.8
  * Using a virtual environment: yes
  
  ### Issue description
  
  I use the `testbot` fixture to run my tests. Since upgrade `err-bot` from `5.2` to `6.1.2`, I see this error when executing the tests in my environment:
  
  ```
  Traceback (most recent call last):
    File ".env36\Lib\site-packages\errbot\backends\graphic.py", line 18, in <module>
      from PySide import QtCore, QtGui, QtWebKit
  ModuleNotFoundError: No module named 'PySide'
  2020-01-30 09:33:24,857 CRITICAL errbot.plugins.graphic     To install graphic support use:
      pip install errbot[graphic]
  ```
  
  It seems the default for backends has changed to "graphic". If I install `errbot-5.2` then my tests pass as expected.
  
  My question is: how do I change the backend of `testbot` to `text`?
  
  Also, shouldn't the backend for `testbot` be `text` by default?
  