Create initial unit test for `PhasorDynamics::Genrou` model

[pelesh]

Implement unit tests for PhasorDynamics::Genrou model, using the existing unit test placeholder. Implement tests for

• constructor
• residual evaluation
• initialization

Unit tests do not need to run any simulation, they just need to verify for correctness methods in PhasorDynamics::Genrou class.

[pelesh]

I suggest taking a look at the branch unit tests as an example how to address this issue.

CC @tsybinae