test: remove leaky unmount from App test

Jest reported it to be causing an open handle at the end of the test suite

src/App.spec.tsx

@@ -5,9 +5,7 @@ import App from './App'
 describe( 'App', () => {
   describe( 'Initial load', () => {
     it( 'Renders without crashing', async () => {
-      const { unmount } = render( <App /> )
-
-      unmount()
+      render( <App /> )
     } )
   } )
 } )