title: create-react-app testing "React is not defined"
body: <p>I am using <code>react-scripts@2.1.8</code> with <code>typescript@3.3.3</code> with <code>jest@.23.6.0</code> and running my tests, they fail with "React is not defined".</p>
  
  <pre><code> FAIL  src/components/Button/test.tsx
    ● &lt;Button&gt; › renders text correctly
  
      ReferenceError: React is not defined
  
         6 |     it('renders text correctly', () =&gt; {
         7 |         const text = 'hello there i am a test'
      &gt;  8 |         const { getByText } = customRender(&lt;Button&gt;{text}&lt;/Button&gt;)
           |                                            ^
         9 |         expect(getByText(/hello there i am a test/)).toBeTruthy()
        10 |     })
        11 |     it('matches the snapshot', () =&gt; {
  
        at Object.it (src/components/Button/test.tsx:8:44)
  
    ● &lt;Button&gt; › matches the snapshot
  
      ReferenceError: React is not defined
  
        10 |     })
        11 |     it('matches the snapshot', () =&gt; {
      &gt; 12 |         const { container } = customRender(&lt;Button /&gt;)
           |                                            ^
        13 |         expect(container.firstChild).toMatchSnapshot()
        14 |     })
        15 | })
  
        at Object.it (src/components/Button/test.tsx:12:44)
  </code></pre>
  
  <p>Button.test.tsx:</p>
  
  <pre><code>import React from 'react'
  import Button from '.'
  import { customRender } from '../../test-utils'
  
  describe('&lt;Button&gt;', () =&gt; {
      it('renders text correctly', () =&gt; {
          const text = 'hello there i am a test'
          const { getByText } = customRender(&lt;Button&gt;{text}&lt;/Button&gt;)
          expect(getByText(/hello there i am a test/)).toBeTruthy()
      })
      it('matches the snapshot', () =&gt; {
          const { container } = customRender(&lt;Button /&gt;)
          expect(container.firstChild).toMatchSnapshot()
      })
  })
  </code></pre>
  
  <p>Also playing with React import, like moving it from line 1 to line 3 seems to sometimes make the tests pass. Which is very strange.</p>