Replace cpu-benchmark with similar stress-ng monte-carlo test

[quantumsteve]

First attempt at replacing cpu-benchmark with a nearly identical test in stress-ng.

differences

• cpu-benchmark uses a full sphere, while stress-ng uses a single quadrand.
• cpu-benchmark uses a "terrible" but efficient rng while stress-ng has many options. I chose "lcg" to start with.
• cpu-benchmark batches cpu-work into 1000000 samples, while stress-ng uses ~16384 samples.
• stress-ng defines samples and ops as int32_t, while samples in cpu-benchmark are int64_t

stress-ng launches multiple processes, which changes the logic for stopping parent and child processes. A quick search recommended psutil.