Replace Enzyme with React Testing Library

[taneliang]

React Testing Library was added in #3007 so that it's easier to test components in context and from a user's perspective. It also feels like a much nicer API.

To avoid having 2 test libraries around (and the associated confusion and the effort needed to learn both), we'll want to replace Enzyme with React Testing Library. There are >30 remaining test files that use enzyme.

This issue tracks the removal of Enzyme from the codebase.

Work to resolve this should probably be done over multiple PRs as it's a rather large task. It may not be trivial since some of our tests test component implementation details and would need to be overhauled.

It may be best to see how React Testing Library is currently used in the codebase (and on the Internet, because we may be doing things wrong) and following that. There are some test utilities in our codebase that may help.