Expanding mutation test

[mmikita95]

Adding a class method to count initial mutations for a StateProxy. Checking the amount of mutations per each step of the test to ensure that mutations are properly tracked.

In regards to our prior discussion with @ramedina86, I found that the most reliable way to avoid issues like #198 in the future is to consistently track how many mutations are produced by the get_mutations_as_dict method in StateProxy, and compare the amount to the "ground truth" (i.e. actual number of mutations, which is easily deductible). I updated the mutation test to do exactly that. A method that counts expected "initial mutations" was also added, to make the test independent from the state of the dictionary initially passed to TestStateProxy.

I will be considering designing other tests as well, and any feedback is appreciated on the way.

[ramedina86]

Nice tests! I'm not a hardcore test fanatic, but for these specific matters they're really helpful.