Create Diplomacy specific unit tests for stance

[DenisPeskoff]

-learn pytest
-scope existing tests
-write abstract logic tests
-figure out who would be helpful for implementation in our code base

[aphedges]

@DenisPeskov posted this possible test on 2023-02-27 in Slack:

Beginning of game:
1901 Austria moves from Tri into Italian Ven.
This should cause Italian stance to change from neutral to very hostile.
It should also cause (or be that case that) Austria to cause stance to be hostile for Italy.

This can contrast with Austria moving Tri away from Italy (into Alb) which should cause Italy to retain a neutral or maybe even have a positive outlook on Austria since Austria moves away from Italys border