Add react-testing-library to project devDependencies

[pjfitzgibbons]

Is your feature request related to a problem? Please describe.

Testing enhancements like waitFor will improve the overall quality of the test suite, even making some tests feasible that are otherwise not possible or very difficult without react-testing-library.

Describe the solution you'd like

yarn add -D react-testing-library

Describe alternatives you've considered

See #1193, split_button.test.tsx:37

Additional context

Add any other context or screenshots about the feature request here.