[BD-5145] Add testing tabs and redirects

[rachel-feinberg]

Why are you making this change? (required)

Added missing messaging channels to the testing tabs and added missing "Testing" redirect headers to the respective messaging channel sections.

Related PRs, issues, or features (optional)

BD-5145

Feature release date (optional)

• N/A

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[github-actions]

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• ⚖️ @ats-91 - PR diff contains privacy/legal/GDPR-related content