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/Components/Cell.cs

@@ -0,0 +1,13 @@
+using MessagePack;
+
+namespace Game.TicTacToe;
+
+/// <summary>
+/// Identifies a board position. One entity per cell.
+/// </summary>
+[MessagePackObject]
+public struct Cell
+{
+    [Key(0)] public int Row;
+    [Key(1)] public int Col;
+}

Game.TicTacToe/Components/Mark.cs

@@ -0,0 +1,13 @@
+using MessagePack;
+
+namespace Game.TicTacToe;
+
+/// <summary>
+/// A mark placed on a cell by a player.
+/// Only present on cells that have been claimed.
+/// </summary>
+[MessagePackObject]
+public struct Mark
+{
+    [Key(0)] public Player Player;
+}

Game.TicTacToe/Components/Player.cs

@@ -0,0 +1,8 @@
+namespace Game.TicTacToe;
+
+public enum Player : byte
+{
+    None = 0,
+    X = 1,
+    O = 2
+}