Improve ExtendedAssertion setup

[dkozma]

Right now, ExtendedAssertions seem a bit unclear and fragile - it would be nice to:

• Improve this setup so that it works in all cases
• Provide documentation so that it is clear to use

[dkozma]

@adobe export issue to Jira project CAI

[github-jira-sync-bot]

:white_check_mark: Jira issue https://jira.corp.adobe.com/browse/CAI-4347 is successfully created for this GitHub issue.