Python api unit tests for Result (#6763)

Python api unit tests for Result (#6763)

This PR translates the cpp API unit tests about results to python.
The original cpp file is: https://github.com/cvc5/cvc5/blob/master/test/unit/api/result_black.cpp

The translation made rise to one addition to the python API:
The UnknownExplanation object from the cpp API was represented by a string in the python API.
Now we have a more faithful representation, as an enum.