fix: removes duplicate id in text track settings

[mister-ben]

Description

Prevents a duplicate id in text track settings.
Fixes #8754

Specific Changes proposed

The text edge settings were erroneously duplicating the id generated for the background colour. Changes to a unique name.
Adds a general test for duplicate ids.

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
  • Docs/guides updated
  • Example created (starter template on JSBin)
  • Has no DOM changes which impact accessiblilty or trigger warnings (e.g. Chrome issues tab)
  • Has no changes to JSDoc which cause npm run docs:api to error
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.01%. Comparing base (8729e31) to head (da1b6dd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8755   +/-   ##
=======================================
  Coverage   83.01%   83.01%           
=======================================
  Files         119      119           
  Lines        7993     7993           
  Branches     1922     1922           
=======================================
  Hits         6635     6635           
  Misses       1358     1358           

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