chore: Add test coverage for screen resize handling

[jperals]

Description

Moving these tests out of the demos, because handling screen resize is a responsibility of the board components library and not of the consumers' board implementation, and also because these tests are causing flakiness (AWSUI-61284).

Related links, issue #, if available: AWSUI-61284

How has this been tested?

Ran the new tests locally.

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 95.11%. Comparing base (219c285) to head (4661d53).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #381   +/-   ##
=======================================
  Coverage   95.11%   95.11%           
=======================================
  Files          67       67           
  Lines        3278     3278           
  Branches      702      702           
=======================================
  Hits         3118     3118           
  Misses        160      160           

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