Revisit aria_landmark_name_unique rule logic for "regions" duplications

[philljenkins]

Originally reported by @David-Yuen

Related to #2157 that was previously closed for "Search" cases.

Triggered by
Rule ID: aria_region_label_unique
Reason ID: Fail_1
Violation
Multiple elements with "region" role do not have unique labels
Each element with "region" role must have a unique label that describes its purpose

reproduce via

1. Test page: https://githubuobskza7-51hx--5173--d20a0a75.local-credentialless.webcontainer.io/
2. Clicked the "Maximize" button on the top right, this opens a Tearsheet modal window with the same instance of the Table.
3. Scan page, the rule above throws incorrect non-unique label error

[philljenkins]

Before fix in #2157, there were two region label errors incorrectly triggered, both a "search" and a "region":

After fix, only one region label is incorrectly triggered, a "region" label.

• Hint: may this be due to the use of the "rare" <section... element?

[shunguoy]

triage: 1. nested landmarks; 2. need an SME discussion for a consensus on handling this: should names for all the landmarks be unique? https://www.w3.org/WAI/ARIA/apg/patterns/landmarks/examples/region.html