Implement pyuci mocker

The idea is to have pure python package as part of pyuci that can be used the same way as uci so those projects using pyuci do not have to pull in to test framework whole UCI with its dependencies. This means libuci and libubox.

Implementation is going to be oriented for usage with pytest and will mock all API while allowing manipulation with "stored" configuration.

• change test framework so it allows us to run it on both real uci and fake one
• implement memory only backend for fake uci
• implement mockup uci for new memory only backend