Add tests for bot_plugin.py

[ifosch]

Best approach to test this would probably be creating a BotPlugin subclass with CommandHandlers and ensure these:

• Are added when using the add_handlers method, using a fake dispatcher
• Get listed using commands
• Are removed when using the remove_handlers method, using a fake dispatcher

Features to test:

• Add handlers
• Remove handlers
• List commands

Fakes/spies to be implemented:

• Dispatcher