fix: Resolves captions sizing issue when minified

[mister-ben]

Description

Fixes issue with sizing captions in #8425.

Specific Changes proposed

Changes test to check for presence of getCueAsHTML instead of the constructor name, as the constructor name is changed when minified.
Open to suggested for a test for this, since the issue only manifested when minified.

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)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #8442 (6d4e21b) into main (b95cd7a) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #8442   +/-   ##
=======================================
  Coverage   82.68%   82.68%           
=======================================
  Files         113      113           
  Lines        7578     7578           
  Branches     1821     1821           
=======================================
  Hits         6266     6266           
  Misses       1312     1312           

Files Changed	Coverage Δ
src/js/tracks/text-track.js	94.23% <100.00%> (ø)

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[wseymour15]

Looks good! Thanks for updating this.