Mk/Uses/cmake.mk: use proper environment in do-test

• the actual test run used MAKE_ENV but should use TEST_ENV
• also, ctest(1) defaults to running tests serially, so add CTEST_PARALLEL_LEVEL here and set it to _MAKE_JOBS_NUMBER. NOTE: cmake 3.29 changes semantics for _MAKE_JOBS_NUMBER empty or 0.

This was developed together with diizzy@ as a result of analysing
performance regressions from PR 278500.

PR: 278533
Approved by: portmgr@ (tcberner@)