Create `Writing Playwright tests` in `TESTING.md`

[lukasbriza]

The aim of this issue is to create section Writing Playwright tests in TESTING.md as level 2 heading.

Documentation should contain rules how to test, what to test, formatting of tests and so. Please, review the list of requirements from https://github.com/react-ui-org/react-ui/issues/587 and use necessary items and reword them to match the rest of the documentation.

Also review pull requests https://github.com/react-ui-org/react-ui/pull/608 and https://github.com/react-ui-org/react-ui/pull/588 and create points of documentation from them.

While the documentation should contain template how to write tests with all describe blocks, do not forget to include formatting tips like default props first, then alphabetically ordered props (if there are multiple propTests like feedback color, neutral color and so, it is still color props, so order it alphabetically as color.