Run some benchmarks measuring the CPU time

[maflcko]

For some benchmarks it would be nice to measure the consumed CPU time.

E.g.

• ibd/reindex, which could be dominated by disk/net access.
• An "idle test", where bitcoind in idle mode should not consume cpu (see https://github.com/bitcoin/bitcoin/issues/16004)

I think this can be achieved by evaluating the formatted string (%U + %S) from time.