Disallow possessing player controlled soldier

[Leroy231]

• [x] On map we should just hide the possess button in this case. Note this may already be implemented, first verify.
• [x] Fix UPossessSoldierEntity_AsyncAction::FindFirstAvailableSoldierEntity to skip soldiers who are already possessed (can check UMilitaryUnit.bIsPlayer)
• [ ] In separate PR: add GitHub action step to run new functional tests. In test plan show that action fails when test fails and action passes when action passes. You can test on a branch before PR here using "Run workflow" button: https://github.com/HaywireInteractive/OnAllFronts/actions/workflows/build-project.yml