Test improvements

[acelaya]

This PR introduces these improvements in tests:

• Use content: () => <MyComponent /> over content: () => mount(<div><MyComponent /></div>) in accessibility tests for components with a top-level Fragment, where the wrapping div was a workaround to prevent some children from being "lost" (see https://github.com/hypothesis/client/issues/5690)
• Fix a warning printed in ToastMessages test that could make debugging other issues harder.
  

[codecov]

Codecov Report

Merging #5914 (c5b2ffb) into main (9b11269) will not change coverage.
The diff coverage is n/a.

❗ Current head c5b2ffb differs from pull request most recent head 41f756d. Consider uploading reports for the commit 41f756d to get more accurate results

@@           Coverage Diff           @@
##             main    #5914   +/-   ##
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  Coverage   99.44%   99.44%           
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  Files         257      257           
  Lines        9880     9880           
  Branches     2360     2360           
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  Hits         9825     9825           
  Misses         55       55           

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