Allow tests to be run in parallel (CMT/old cmake)

Currently, tests with old CMake or CMT do not respect the --jobs parameter of the script (defaulting to the LBN_BUILD_JOBS environment variable, or 1 if not set). This MR removes this hardcoded behaviour.

There will be no immediate effect on the tests according to the the current configuration in Jenkins (LBN_BUILD_JOBS not set for tests).

The idea behind this is that @sponce and I would like to run tests in parallel (to reduce time until slot completion). We will try to solve/workaround any concurrency issues we see in the various slots.

Closes #78 (closed)