add test for destroy camera

thermion_dart/test/camera_tests.dart

@@ -9,6 +9,13 @@ void main() async {
   final testHelper = TestHelper("camera");
   await testHelper.setup();
   group('camera', () {
+    test('create/destroy camera', () async {
+      await testHelper.withViewer((viewer) async {
+        final camera = await viewer.createCamera();
+        await camera.destroy();
+      });
+    });
+
     test('model matrix', () async {
       await testHelper.withViewer((viewer) async {
         final camera = await viewer.getActiveCamera();
@@ -21,7 +28,6 @@ void main() async {
         expect(position.x, 2.0);
         expect(position.y, 2.0);
         expect(position.z, 2.0);
-
       });
     });