test: fix flaky `issue-comment` close test (#37880)

After posting a comment the page reloads via fetch-action. Clicking
"Close Issue" before the form re-initializes triggers a native form
submit, which navigates to the raw JSON redirect response
(`{"redirect":...}`) instead of the issue, so "Reopen Issue" never
appears and the test times out (observed on Firefox in CI).

Wait for the comment button to become disabled — which only happens once
the form re-initializes — before clicking "Close Issue".

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This PR was written with the help of Claude Opus 4.7

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: Nicolas <bircni@icloud.com>