Switch test and subtest timing to use high precision real time and cpu timers

@chrisdembia  pointed out in PR #253 that Simbody's `Test` class timing counted only CPU time and therefore was no good for checking if multithreading worked properly.

`Test` was using the ancient `std::clock()` timer. I switched it to use Simbody's high precision real time and CPU time functions. This required some formatting changes for the output -- please check if they are reasonable.