Testing documentation update

[HonzaR]

Is your feature request related to a problem? Please describe.

We'd like to have our approach to testing better documented.

Describe the solution you'd like

Ideally, it'd be one docs/testing/Testing.md file, which outlines possibly all corners of how we test the sdk:

• automated tests (unit x instrumented)
• integration tests
• manual tests
• tests run on CI
• benchmark tests (with last results section)
• screenshot tests
• code coverage
• etc.