test: add unit tests for Blackjack and TicTacToe

- Add xUnit test projects for both games
- Implement game flow tests for Blackjack
- Implement game flow tests for TicTacToe
- Rename namespaces and projects to follow `Game.*` convention
- Add documentation for testing games

Game.TicTacToe/Singletons/GameState.cs

@@ -0,0 +1,14 @@
+using MessagePack;
+
+namespace Game.TicTacToe;
+
+/// <summary>
+/// Global game state stored on the singleton entity.
+/// </summary>
+[MessagePackObject]
+public struct GameState
+{
+    [Key(0)] public Player CurrentPlayer;
+    [Key(1)] public GameStatus Status;
+    [Key(2)] public int MoveCount;
+}

Game.TicTacToe/Singletons/GameStatus.cs

@@ -0,0 +1,9 @@
+namespace Game.TicTacToe;
+
+public enum GameStatus : byte
+{
+    Playing = 0,
+    XWon = 1,
+    OWon = 2,
+    Draw = 3
+}