Improve documentation regarding tracing in tests

[mhalbritter]

Tracing in tests is disabled, as documented here.

We got a lot of questions why tracing isn't working in tests, so we should document this more prominently.

We should consider:

• documenting it in the tracing section
• add a logger call in the component which disables tracing in tests with a message pointing to the documentation or how to enable it

[dsyer]

Please also consider pulling the whole "Testing" section up to the top level so it shows up in the left nav.