Add a new JCTEST variable to cmake that allow for running the tests in
parallel using multiple jobs. The variable is cached by cmake, so
running ninja multiple times will use the same variable value. Default
is to run a single job.

Even if this is not strictly a dependency, running the test plan is more
efficient if done on top of D2496.

mkdir buildcmake && cd buildcmake

Build and run tests on 1 job:

cmake -GNinja .. && ninja -j1 check

Build on 4 jobs and run tests on 1 job:

cmake -GNinja .. && ninja -j4 check

Build on 4 jobs and run tests on 2 jobs:

cmake -DJCTEST=2 -GNinja .. && ninja -j4 check

Build on 6 jobs and run tests on 2 jobs:

ninja -j6 check