Add PROCESSORS specification when creating tests

[bartgol]

When using add_test, if we set the PROCESSORS property, we can more efficiently use the parallel execution of ctest.

To run multiple tests concurrently, one should do CTEST_PARALLEL_LEVEL=N ctest. However, without setting the PROCESSOR property, cmake/ctest thinks that each test uses 1 processor, so with N=2 you'd be running two tests concurrently, even if they are MPI tests, using several ranks. Ideally, one wants to set N to be the number of available resources, and let ctest figure out how many tests to run concurrently. I sometimes forget what I set the max number of ranks to be, and don't really want to go check, to figure out how many tests I can run concurrently...