Use consistent test style across codebase

In https://github.com/testing-library/user-event/pull/558 discussions, we've noticed that the test files across the codebase have a mix of declarative and imperative style.

What happened: When contributing for the first time, I didn't know what style to follow while coding the tests since it's mixed across the codebase and there's tool/docs mention to enforce it

The following code is from __tests__/paste.js

// declarative
test('should paste text in textarea', () => { ... }

// imperative
test('does not paste when readOnly', () => { ... }

Reproduction repository: Go through the tests file of the codebase and see a mixture of declarative and imperative styles.

Problem description: Contributors should know whether style to choose, the current situations may cause a lot of confusion and PR discussions

Suggested solution: Discuss in this issue the following topics:

1. What would be the best test style for this codebase: Declarative or Imperative?
2. How to enforce a test style?

Begin with tests/paste.js and the discussion in pull request #558, then review surrounding test files to compare the two styles. Done means the project has agreed on one style and documented or enforced it for contributors; the issue does not name a specific enforcement file or test.