[ui-components] Replace enzyme with react-testing-library

[laurenbarker]

What

Replace enzyme with react-testing-library for tests in ui-components.

Why

We have been piloting React Testing Library in Console and have had a very positive experience. It encourages good test practices and is a lot simpler to use than enzyme.

[laurenbarker]

@j-low what do you think of switching to RTL for ui-components?

[tharun208]

@laurenbarker, I refactored the Avatar component tests using RTL #227. And refactoring other components unit tests will come as a separate pr.