New comment by Logarithmus on void-packages repository

https://github.com/void-linux/void-packages/pull/31811#issuecomment-879974474

Comment:
> Should we add `disable_parallel_tests` to override this? Some test suites are explicitly broken when run in parallel (iirc cups, for example).
> 
> I assume they left their approval so others could review.

I've added `disable_parallel_check` option and tested it for `mold`, it works as intended.

A little TODO:
- document the new option in `Manual.md`
- try running tests for `cups*` packages and see if it fails without `disable_parallel_check`.