Add Timer tests and fix progress (#611)

* Add Timer tests and fix progress

* Add Timer tests & progress fix to CHANGELOG

test/timer_test.dart

@@ -0,0 +1,90 @@
+import 'package:flame/timer.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Timer', () {
+    test('can be started and stopped, discarding progress', () {
+      final timer = Timer(1.0);
+      expect(timer.isRunning(), false);
+      timer.start();
+      expect(timer.isRunning(), true);
+      timer.update(0.5);
+      timer.stop();
+      expect(timer.isRunning(), false);
+      expect(timer.current, 0.0);
+    });
+
+    test('can be paused and resumed, retaining progress', () {
+      final timer = Timer(1.0);
+      expect(timer.isRunning(), false);
+      timer.start();
+      expect(timer.isRunning(), true);
+      timer.update(0.5);
+      timer.pause();
+      expect(timer.isRunning(), false);
+      timer.resume();
+      expect(timer.isRunning(), true);
+      expect(timer.current, 0.5);
+    });
+
+    test('tracks current delta time', () {
+      final timer = Timer(1.0);
+      timer.start();
+      timer.update(0.5);
+      expect(timer.current, 0.5);
+      timer.update(0.2);
+      expect(timer.current, 0.7);
+    });
+
+    test('tracks progress percent capped at 1.0', () {
+      final timer = Timer(2.0);
+      timer.start();
+      timer.update(0.5);
+      expect(timer.progress, 0.25);
+      timer.update(0.5);
+      expect(timer.progress, 0.5);
+      timer.update(9999);
+      expect(timer.progress, 1.0);
+    });
+
+    test('callback fires once if non-repeating', () {
+      var callbackCount = 0;
+      final timer = Timer(1.0, callback: () => callbackCount++);
+      timer.start();
+      timer.update(0.9);
+      expect(callbackCount, 0);
+      timer.update(0.2);
+      expect(callbackCount, 1);
+      timer.update(1.0);
+      expect(callbackCount, 1);
+    });
+
+    test('finishes when complete if non-repeating', () {
+      final timer = Timer(1.0);
+      timer.start();
+      expect(timer.finished, false);
+      timer.update(1.1);
+      expect(timer.finished, true);
+    });
+
+    test('callback fires repeatedly if repeating', () {
+      var callbackCount = 0;
+      final timer = Timer(1.0, repeat: true, callback: () => callbackCount++);
+      timer.start();
+      timer.update(0.9);
+      expect(callbackCount, 0);
+      timer.update(0.2);
+      expect(callbackCount, 1);
+      timer.update(1.0);
+      expect(callbackCount, 2);
+    });
+
+    test('does not finish past limit if repeating', () {
+      final timer = Timer(1.0, repeat: true);
+      timer.start();
+      expect(timer.finished, false);
+      timer.update(1.1);
+      expect(timer.finished, false);
+    });
+  });
+}