add test

Author: li-jia-nan

tests/__snapshots__/index.test.tsx.snap

@@ -0,0 +1,9 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`StrictMode 1`] = `
+<div>
+  <button>
+    click
+  </button>
+</div>
+`;

tests/index.test.tsx

@@ -28,3 +28,13 @@ describe('MutateObserver', () => {
     unmount();
   });
 });
+
+it('StrictMode', () => {
+  const { container } = render(
+    <MutateObserver>
+      <div>test</div>
+    </MutateObserver>,
+    { wrapper: React.StrictMode },
+  );
+  expect(container).toMatchSnapshot();
+});