Unflake sandboxed_mermaid_spec

What does this MR do and why?

Unflake sandboxed_mermaid_spec and dequarantine some specs while we're at it.

wait_for_requests terminates arbitrarily; expect(page.html) is non-waiting. This is strictly better than what we have now.

• Part of Remove all uses of `wait_for_requests`, `wait_f... (gitlab-org#22879).
• Unquarantines https://gitlab.com/gitlab-org/quality/test-failure-issues/-/issues/9494+.
• Unquarantines https://gitlab.com/gitlab-org/quality/test-failure-issues/-/issues/9387+.

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