Migrate to React Testing Library

[christopher-stripe]

#93 adds support for Strict Mode, but we don't have any way to make sure we don't regress this change since Enzyme doesn't support Strict Mode. This PR migrates our usage of Enzyme to React Testing Library, which does support Strict Mode, so that we can confidently merge #93.

A few notes about the migration:

• I opted to test hooks with react-hooks-testing-library where it seemed reasonable
• I converted the tests from JavaScript to TypeScript since it was an easy change to make concurrently, and because I find the type system to be slightly more helpful than irritating in test code. But I am also peaceful keeping tests as JS if you have strong opinions about this
• I also added a few failing tests for Strict Mode, which are currently skipped

[christopher-stripe]

This doesn't actually test strict mode 🤦‍♀️

[dweedon-stripe]

Thanks for doing this!

[dweedon-stripe]

nit: I get how this works, but it so badly breaks the React model that if feels like it should not. Can this be a bit more idiomatic?

[christopher-stripe]

Yeah, does feel a bit weird (though it is what's prescribed in the React Hooks Testing Library). Because you expect that props should not be mutated outside of the React lifecycle, right?

Would this be preferable?

  test('allows a transition from null to a valid Stripe object', () => {
    const TestComponent = () => {
      const elements = useElements();

      if (!elements) {
        return null;
      }

      if (elements === mockElements) {
        return <>expected</>;
      }

      throw new Error(`unexpected useElements value: ${elements}`);
    };

    const {container, rerender} = render(
      <Elements stripe={null}>
        <TestComponent />
      </Elements>
    );

    expect(container).toBeEmpty();

    rerender(
      <Elements stripe={mockStripe}>
        <TestComponent />
      </Elements>
    );

    expect(container).toHaveTextContent('expected');
  });

[christopher-stripe]

Updated this and the next, lemme know what you think.

[dweedon-stripe]

Same thing here.

[dweedon-stripe]

This actually seems overly round-about. The way you had it before is fine once I get my head around it. Especially if the old way is the blessed way of testing.

[christopher-stripe]

Cool, I'll leave it the original way then