Cleanup data-page attribute after each Karma test

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3115

• Cleanup data-page attribute after each Karma test to avoid leaking into the next test

Are there points in the code the reviewer needs to double check?

• Are we okay with moving to consistent .attr('data-page', ... for easier Ctrl+F?
  • Nope, moving back to .data('page')
  • Yes, $.attr actually changes the data attribute value and keeps things consistent with existing data-page from HAML

Why was this MR needed?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/38871

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #38871 (closed)