test: split test-embedding.js and run tests in parallel

[joyeecheung]

Split the tests so that it's easier to isolate and debug issues, and we can run the test cases in parallel to speed up. Also add more comments about what they are testing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.78%. Comparing base (6cc2ed5) to head (9073096).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #61571   +/-   ##
=======================================
  Coverage   89.77%   89.78%           
=======================================
  Files         673      673           
  Lines      203820   203820           
  Branches    39186    39173   -13     
=======================================
+ Hits       182982   182990    +8     
+ Misses      13155    13153    -2     
+ Partials     7683     7677    -6     

see 29 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71116/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71126/