Add custom attribute to custom element in test

[scpeters]

🦟 Bug fix

Confirms that #54 has been fixed

Summary

Issue #54 was opened before we had documented support for custom elements and attributes and mentions that custom attributes are not supported within a custom element. This was fixed at some point (not sure when), and I have confirmed this by adding a custom attribute inside a custom element in the test/integration/custom_elems_attrs.sdf file and updated the associated expectations in the INTEGRATION_sdf_custom test.

Checklist

• [X] Signed all commits for DCO
• [X] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [X] codecheck passed (See contributing)
• [X] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov[bot]]

Codecov Report

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

Project coverage is 92.48%. Comparing base (ff9b3ad) to head (a9024fa). Report is 33 commits behind head on sdf14.

Additional details and impacted files

@@            Coverage Diff             @@
##            sdf14    #1406      +/-   ##
==========================================
+ Coverage   92.42%   92.48%   +0.05%     
==========================================
  Files         134      135       +1     
  Lines       17751    18072     +321     
==========================================
+ Hits        16406    16713     +307     
- Misses       1345     1359      +14     

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