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 #587 and use necessary items and reword them to match the rest of the documentation.

Also review pull requests #608 and #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.

[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 #587 and use necessary items and reword them to match the rest of the documentation.

Also review pull requests #608 and #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.

As I started writing documentation on commit a7d1e91b617142be8ca66800cf82a55a0f28572a where is no TESTING.md file due to update of symlinks i decided to create new file in docs/contribute named writing-tests-guidlines.md