Make "user edits issue" labels feature spec less flaky

What does this MR do and why?

Rely on Capybara's "wait for element" and remove some unneeded wait_for_requests calls and clarifies the expectation.

Closes #338646 (closed).

Screenshots or screen recordings

For more much details see #338646 (comment 717337014).

How to set up and validate locally

while :; do be rspec spec/features/issues/user_edits_issue_spec.rb:144 || break; done
# wait for it fail

MR acceptance checklist

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