Improve the testing documentation
What does this MR do?
This improves our current "Testing" documentation:
-
Document the different levels of testing and how to test at the correct testing level -
Document good practices for System / Feature tests -
Document how the test suite is parallelized on the CI -
Document that we monitor our test suite -
Document gitlab-qa -
Document the new spec/architecture: #29627 (moved)
What are the relevant issue numbers?
Closes #29499 (closed), #29623 (closed).