Implement test for No. 4

tests/station04.test.tsx

@@ -0,0 +1,46 @@
+import React from 'react'
+import renderer, { act } from 'react-test-renderer'
+import { App } from '../src/App'
+
+
+describe('<App />', () => {
+  let setState: React.Dispatch<unknown> | undefined = undefined
+  const useState = React.useState
+  const useStateSpy = jest.spyOn(React, 'useState')
+  useStateSpy.mockImplementation((v?: unknown) => {
+    const [value, dispatcher] = useState(v)
+    setState = dispatcher
+    return [value, dispatcher]
+  })
+
+  afterEach(() => {
+    jest.clearAllMocks()
+  })
+
+  it('<App /> calls useState', () => {
+    renderer.create(<App />)
+
+    expect(useStateSpy).toBeCalled()
+  })
+
+  it('<img> uses a state value', () => {
+    const res = renderer.create(<App />)
+    const img = res.root.findByType('img')
+    const injectValue = '🐕'
+
+    expect(img.props.src).toBeTruthy()
+    expect(useStateSpy).toBeCalledWith(img.props.src)
+
+    useStateSpy.mockClear()
+
+    expect(setState).not.toBeUndefined()
+
+    act(() => {
+      if (setState) {
+        setState(injectValue)
+      }
+    })
+
+    expect(img.props.src).toBe(injectValue)
+  })
+})