React tests

[beaucollins]

In preparation for fixing #22 this sets up tests for the existing implementation.

Switches from enzyme to use @testing-library/react which is designed to be able to test component behavior that relies on hooks (like React.useEffect).

How to Test

npm test
 PASS  src/components/FlatfileButton.test.tsx
  <FlatfileButton settings={testSettings} />
    ✓ renders button (69 ms)
    ✓ calls requestDataFromUser when button is clicked (9 ms)

Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        1.4 s, estimated 3 s
Ran all test suites.

Coverage

--------------------|---------|----------|---------|---------|----------------------------------
File                | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                
--------------------|---------|----------|---------|---------|----------------------------------
All files           |   72.55 |    30.77 |   44.44 |   71.43 |                                  
 FlatFileButton.tsx |   72.55 |    30.77 |   44.44 |   71.43 | 62,66-67,71,74-84,95-102,107,111 
--------------------|---------|----------|---------|---------|----------------------------------