Refactor test utilities

[kettanaito]

There are several points about our test utilities (test/helpers.ts) that I find unpleasant:

1. Introduce too much logic and, respectively, boilerplate to be used.
2. The concept of pool and automatic lookup is terrible. Use jest.fn() instead!
3. All written in a single, large file, making them hard to read.

Expected behavior

1. Get rid of utilities that are written only for the sake of promisifying callback-based request modules (i.e. http/XMLHttpRequest). Embrace those modules directly, this will improve their reliability and increase their referencing factor.
2. Structure the remaining utilities under their respective surface: test/helpers/http.ts, test/helpers/XMLHttpRequest.ts, etc.).
3. Adjust the affected tests.

[kettanaito]

events-order test can also be improved. We can utilize Jest's .toHaveBeenNthCalledWith to assert both the event listener's call order and arguments.

[kettanaito]

Removed the entire test/utils directory that contained what appears the initial implementation of test-server.

[github-actions]

🎉 This issue has been resolved in version 0.13.5 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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