Use StrictMode in integration tests

[nstepien]

 PASS  tests/ReactDataGrid.test.ts
  √ rendering <ReactDataGrid /> should work (61ms)

  console.error node_modules/react-dom/cjs/react-dom.development.js:506
    Warning: Unsafe lifecycle methods were found within a strict-mode tree:
        in StrictMode
    
    componentWillReceiveProps: Please update the following components to use static getDerivedStateFromProps instead: Header, ReactDataGrid, Viewport
    
    Learn more about this warning here:
    https://fb.me/react-strict-mode-warnings

  console.error node_modules/react-dom/cjs/react-dom.development.js:506
    Warning: findDOMNode is deprecated in StrictMode. findDOMNode was passed an instance of HeaderRow which is inside StrictMode. Instead, add a ref directly to the element you want to reference.
    
        in div (created by HeaderRow)
        in HeaderRow (created by Header)
        in div (created by Header)
        in Header (created by Grid)
        in div (created by Grid)
        in Grid (created by ReactDataGrid)
        in div (created by ReactDataGrid)
        in ReactDataGrid
        in StrictMode
    
    Learn more about using refs safely here:
    https://fb.me/react-strict-mode-find-node