Add tests to CSP class (#299)
Add tests to CSP class (#299)

* Add tests to CSP class

Add test for the following methods on CSP class:

* assign
* unassigns
* nconflits
* actions

* Refactor some asserts in test_csp.py

Some asserts were being done in the following way:
assert (X is not None) == True

Now, they are handled in the following way:
assert X

Chipe1/aima-python,jo-tez/aima-python,aimacode/aima-python,SnShine/aima-python,SnShine/aima-python,sofmonk/aima-python,Chipe1/aima-python,jo-tez/aima-python,aimacode/aima-python,sofmonk/aima-python