fix: Ensure aria-labelledby values in track settings are valid

[mister-ben]

Description

Fixes new text track settings, so that aria-labelledby attributes contain only valid attributes. Currently there is one invalid id, and two els with duplicate ids.
Fixes #8708

Specific Changes proposed

Updates text-track-settings.js to set the id property to an empty string where there is no label, only a legend element.
Adds tests to validate aria-labelled by values.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.00%. Comparing base (466fa97) to head (d627211).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8711   +/-   ##
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  Coverage   83.00%   83.00%           
=======================================
  Files         120      120           
  Lines        7996     7996           
  Branches     1926     1926           
=======================================
  Hits         6637     6637           
  Misses       1359     1359           

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[adrums86]

Makes sense to me.