feat: included test for BoardSide

lib/game/components/board_side.dart

@@ -12,6 +12,7 @@ enum BoardSide {
   right,
 }
 
+/// Utility methods for [BoardSide].
 extension BoardSideX on BoardSide {
   /// Whether this side is [BoardSide.left].
   bool get isLeft => this == BoardSide.left;

test/game/components/board_side_test.dart

@@ -0,0 +1,27 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pinball/game/game.dart';
+
+void main() {
+  group(
+    'BoardSide',
+    () {
+      test('has two values', () {
+        expect(BoardSide.values.length, equals(2));
+      });
+    },
+  );
+
+  group('BoardSideX', () {
+    test('isLeft is correct', () {
+      const side = BoardSide.left;
+      expect(side.isLeft, isTrue);
+      expect(side.isRight, isFalse);
+    });
+
+    test('isRight is correct', () {
+      const side = BoardSide.right;
+      expect(side.isLeft, isFalse);
+      expect(side.isRight, isTrue);
+    });
+  });
+}