Add more test coverage

[ewu63]

Description

Unit tests

• [ ] test that the Solution object contains the right info
  • [x] objectives
  • [ ] constraints
  • [ ] variables
  • [ ] can be correctly cold-started (document this!)
• [ ] FD/CS tests

Integration tests

• [ ] MPI tests
  • [ ] fake COMM object works on single proc
  • [ ] optimization in parallel runs fine, with only the root proc running the optimization itself but calling the objective/sensitivity functions on all procs
  • [ ] parallel DV/constraint adding works as expected
• [ ] other optimization problems?
• [ ] FD/CS tests