Mk/Uses/cmake.mk: Make parallel level adjustable for tests

Some projects don't support running tests in parallel
(for example chinese/libchewing) and currently there's no way of
disabling it.

Add CMAKE_TESTING_PARALLEL_LEVEL and default to MAKE_JOBS_NUMBER
Switch to MAKE_JOBS_NUMBER as _MAKE_JOBS_NUMBER is unset if
MAKE_JOBS_UNSAFE is defined.

PR: 279316
Approved by: portmgr (arrowd)