Document how we deal with tests expected to fail

Document the procedure mentioned #7233-note_26. Note that when #10198 (closed) is done, and the Suites feature has landed in in Cucumber, we will have a better way to implement this, so this procedure will just be a temporary solution until then.

Feature Branch: kytv/feature/10281-expected-test-failures

Parent Task: #7233 (closed)

Original created by @anonym on 10281 (Redmine)