test: fix IE11 encrypted VTT tests by using an actual encrypted VTT segment

[gesinger]

Also moves the tests under a new module called "Segment Key Caching."

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #1291 (b5f8450) into main (721e1bf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1291   +/-   ##
=======================================
  Coverage   86.30%   86.30%           
=======================================
  Files          39       39           
  Lines        9842     9842           
  Branches     2293     2293           
=======================================
  Hits         8494     8494           
  Misses       1348     1348           

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