Add tests to bot_plugins.py

[ifosch]

Features to test:

• find_bot(module)
• get_list_from_envitonment(env_var_name)
• iter_namespaces()
• import_plugin_modules()
• BotPlugins.__disabled_plugins(disabled_plugins)
• BotPlugins.command_descs()
• BotPliugins.feature_descs()
• BotPlugins.disable(plugin_name)
• BotPlugins.load_plugin(module_name)
• BotPlugins.enable(plugin_name)
• BotPlugins.load_plugins()
• BotPlugins used as an iterator

Mocks needed:

• os.getenv
• os.path.join
• os.path.dirname

Spies to be implemented:

• importlib.import_module