wpt: run tests in parallel (2.5x speed improvement)

[KhafraDev]

Before:

> time npm run test:wpt

[fetch]: Completed: 1283, failed: 140, success: 1143, expected failures: 140, unexpected failures: 0
[FileAPI]: Completed: 234, failed: 19, success: 215, expected failures: 19, unexpected failures: 0
[mimesniff]: Completed: 1898, failed: 1008, success: 890, expected failures: 1008, unexpected failures: 0

real    0m26.233s
user    0m12.328s
sys     0m4.500s

After:

> time npm run test:wpt

[fetch]: Completed: 1283, failed: 140, success: 1143, expected failures: 140, unexpected failures: 0
[FileAPI]: Completed: 234, failed: 19, success: 215, expected failures: 19, unexpected failures: 0
[mimesniff]: Completed: 1898, failed: 1008, success: 890, expected failures: 1008, unexpected failures: 0

real    0m10.181s
user    0m13.641s
sys     0m8.031s

[codecov-commenter]

Codecov Report

Base: 89.55% // Head: 89.55% // No change to project coverage 👍

Coverage data is based on head (50ed1c8) compared to base (dfee69f).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1714   +/-   ##
=======================================
  Coverage   89.55%   89.55%           
=======================================
  Files          58       58           
  Lines        5257     5257           
=======================================
  Hits         4708     4708           
  Misses        549      549           

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[mcollina]

lgtm