This ticket will ensure that Mac testing on Evergreen defaults to consistently using half the logical cores as the number of tests to simultaneously execute with the '-j' option.

On Intel Macs, the number of logical cores is double the number of physical cores because of hyper-threading. So a 6 core Intel CPU will have 12 logical cores.

The Intel and M1-based Mac Minis uses for testing also have limited RAM, so running many tests in parallel increases the risk of running short of RAM

Python unit tests have been using half the local cores to reduce the chance of overloading the system. This ticket will apply that more broadly, in particular to the CTest tests run in 'make check all'