chore(skill): Add skill for writing unit and E2E tests (#20561)

Add skill for writing tests. Created and evaluated with
https://github.com/anthropics/skills/blob/main/skills/skill-creator/SKILL.md

Used sources for "how to write good tests":
- https://vitest.dev/guide/learn/writing-tests-with-ai.html
- https://github.com/andredesousa/javascript-unit-testing-best-practices
- my knowledge from university - LOL

Author: s1gr1d