Update tests from fireEvent to userEvent

[brookewp]

What?

Migrate fireEvent usages to @testing-library/user-event.

In this case, I decided to start by looking at instances of fireEvent.click as the PR would be too large if all instances of fireEvent were included.

And hopefully, by addressing these by method it will be helpful for others trying to migrate from fireEvent and learn about which methods to move to.

Why?

The user-event library provides a more realistic simulation of how a user interacts with the DOM and is the preferred method according to Testing Library docs and our own docs:

https://github.com/WordPress/gutenberg/blob/trunk/docs/contributors/code/testing-overview.md#user-interaction

How?

Testing Instructions

Make sure all tests pass by running npm run test:unit [file-name] for the following:

npm run test:unit packages/components/src/custom-select-control/test/index.js

npm run test:unit packages/components/src/item-group/test/index.js

npm run test:unit packages/components/src/slot-fill/test/slot.js

[brookewp]

Just to note, I removed changes/comments that were no longer relevant due to the React 18 migration and improvements of tests.

[tyxla]

LGTM 👍

There is a console.warn warning in the console, but that's expected and it is present in trunk as well, likely occurring because we're using @wordpress/warning for that warning.

🚀