Update rspec guidelines for aggregating failures

What does this MR do?

Typically when we write tests, we try to only use one expectation per test. However, that isn't always optimal. So in those cases where we have more than one expectation, we should try to use :aggregate_failures in order to see all the failures in a test block at once and not have to piecemeal through it as we get each to pass.

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

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• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

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