Use pest to simplify test maintenance

[spawnia]

As suggested by @vhenzl in #1075 (comment), using https://pestphp.com might ease maintenance of tests. Its syntax is heavily inspired by Jest, which the reference implementation uses.

[ruudk]

Please don't do this and keep using PHPUnit, the tool that everyone uses. Having a test suite that everyone can work with makes it easier for newcomers to participate in the project.

We all know PHPSpec and how that turned out. Some projects still use it and it's a pain to participate in.