Add end-to-end test for issue board focus mode

What does this MR do?

Closes gitlab-org/quality/testcases#182 (closed).

Note: I'm not testing that the top and left navigation bars are not visible when the board is focused due to them being actually visible in the DOM in such case

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content