test: Clean up warnings. Fixes #443

[athill]

Fixes the following warnings when testing:

• Warning: ReactDOMTestUtils.act is deprecated in favor of React.act. Import act from react instead of react-dom/test-utils. See https://react.dev/warnings/react-dom-test-utils for more info. -- Fixed by updating React Testing Library
• Warning: Icon: Support for defaultProps will be removed from function components in a future major release. Use JavaScript default parameters instead. - Fixed by switching to default parameters
• Warning: Invalid DOM property class. Did you mean className? -- Fixed by changing class props to className
• Warning: React does not recognize the testAttr prop on a DOM element. If you intentionally want it to appear in the DOM as a custom attribute, spell it as lowercase testattr instead. If you accidentally passed it from a parent component, remove it from the DOM element. -- Fixed by changing testAttr to testattr
• Warning: Failed prop type: The prop label is marked as required in Subnav, but its value is undefined -- Fixed by passing in required prop to tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.35%. Comparing base (0832916) to head (4b839e7). Report is 2 commits behind head on beta.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## beta #446 +/- ## ======================================= Coverage 99.35% 99.35% ======================================= Files 88 88 Lines 1232 1234 +2 Branches 179 179 ======================================= + Hits 1224 1226 +2 Misses 2 2 Partials 6 6 ```

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[iu-deployer]

:tada: This PR is included in version 2.0.0-beta.6 :tada:

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

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