Add lightweight instrumentation 'probes' for unit tests

When profiling the performance of a given code path, certain metrics can provide valuable insight as to how the profiled code will impact system performance. An obvious example is wallclock time, but additional metrics like L2/L3/TLB cache misses, remote NUMA node memory accesses, page faults, instructions per cycle, and cycle counts can paint a more detailed picture.

The idea here is to add a simple mechanism which collects such metrics to the unit test framework.