CTest: Add options to control test process affinity to CPUs

In commit 5fb958fd CTest learned to track the number of processors allocated to running tests in order to balance it against the desired level of parallelism. Extend this idea by introducing a new PROCESSOR_AFFINITY test property to ask that CTest run a test with the CPU affinity mask set. This will allow a set of tests that are running concurrently to use disjoint CPU resources.