tests: benchmark: improve benchmark scalability accuracy

tests: benchmark: improve benchmark scalability accuracy

Testing with a fixed number of loops per-thread only works if the
workload is distributed perfectly across CPUs. For instance, if a lock
is held in the workload (e.g. internally by open() and close()), those
may cause starvation of some threads, and therefore cause the benchmark
to be wrong because it will wait for the slowest thread to complete its
loops.

It is also not good for testing overcommit of threads vs cpus.

Change the test to report the number of loops performed in a given wall
time, and use this to report the average and std.dev. of tracing
overhead per event on each active CPU.

Change the benchmark workload to be only CPU-bound and not generate
system calls to minimize the inherent non-scalability of the workload
(e.g. locks held within the kernel).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I5245f36831875bd9f87854618a4ed0cb31e56a4d