Added test for batch update

tests/test_app.py

@@ -0,0 +1,12 @@
+from textual.app import App
+
+
+def test_batch_update():
+    app = App()
+    assert app._batch_count == 0
+    with app.batch_update():
+        assert app._batch_count == 1
+        with app.batch_update():
+            assert app._batch_count == 2
+        assert app._batch_count == 1
+    assert app._batch_count == 0