RED: test moving forward in all directions

test/mars_rover_test.dart

@@ -62,5 +62,35 @@ void main() {
         expect(rover.direction, equals('N'));
       });
     });
+
+    group('Moving Forward:', () {
+      test('facing North increases Y', () {
+        final rover = Rover(x: 0, y: 0, direction: 'N');
+        rover.moveForward();
+        expect(rover.x, equals(0));
+        expect(rover.y, equals(1));
+      });
+
+      test('facing East increases X', () {
+        final rover = Rover(x: 0, y: 0, direction: 'E');
+        rover.moveForward();
+        expect(rover.x, equals(1));
+        expect(rover.y, equals(0));
+      });
+
+      test('facing South decreases Y', () {
+        final rover = Rover(x: 0, y: 5, direction: 'S');
+        rover.moveForward();
+        expect(rover.x, equals(0));
+        expect(rover.y, equals(4));
+      });
+
+      test('facing West decreases X', () {
+        final rover = Rover(x: 5, y: 0, direction: 'W');
+        rover.moveForward();
+        expect(rover.x, equals(4));
+        expect(rover.y, equals(0));
+      });
+    });
   });
 }