chore: Add test to demonstrate cross-iframe getModalRoot usage

[just-boris]

Description

Add a test to confirm this behavior is supported

Related links, issue #, if available: AWSUI-59828

How has this been tested?

This whole PR is a test

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.21%. Comparing base (b22ec75) to head (1d49437).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2902   +/-   ##
=======================================
  Coverage   96.21%   96.21%           
=======================================
  Files         761      761           
  Lines       21511    21511           
  Branches     7366     7357    -9     
=======================================
  Hits        20697    20697           
  Misses        761      761           
  Partials       53       53           

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