locale: Optimize tst-localedef-path-norm

The locale generation are issues in parallel to try speed locale
generation.  The maximum number of jobs are limited to the online
CPU (in hope to not overcommit on environments with lower cores
than tests).

On a Ryzen 9, the test execution improves from ~6.7s to ~1.4s.

Tested-by: Mark Wielaard <mark@klomp.org>