enable running tests in parallel

[classabbyamp]

• common: add disable_parallel_check
• common/build-style: make do_check run in parallel unless disabled
• Manual.md: document disable_parallel_check

Redux of #31811, incorporating some of the outstanding feedback when it was stale-closed.

Adds -j$XBPS_MAKEJOBS to do_check in several build-styles, with the option to disable it if it causes issues.

I did not change the python3-{modules,pep517} build-styles to opt-out of parallel checks, because they require a 3rd-party library to do it (and it only works with pytest tests). If a python3 package has tests that require python3-pytest-xdist to run, but have issues with parallel tests, parallel behaviour can still be disabled by disable_parallel_check (which would set the xdist equivalent of -j1).

Testing the changes

• I tested the changes in this PR: YES