fix: Makes board item test utils wrappers chainable

[pan-kot]

Description

Board item has slots and those must return ElementWrapper and not ContentWrapper so that it is possible to chain selectors like createWrapper().findBoardItem().findContent().findButton(). That requires respective components selectors to be imported e.g. import "@cloudscape-design/components/test-utils/dom" or import "@cloudscape-design/code-view/test-utils/dom".

How has this been tested?

• New unit tests

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 93.57%. Comparing base (2a12041) to head (37e7506).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #322   +/-   ##
=======================================
  Coverage   93.57%   93.57%           
=======================================
  Files          61       61           
  Lines        4312     4312           
  Branches      742      741    -1     
=======================================
  Hits         4035     4035           
  Misses        275      275           
  Partials        2        2           

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