Run some benchmarks measuring the CPU time #43
Description
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 Unusual CPU usage on Bitcoin daemon over Tor (addcon, opencon) bitcoin/bitcoin#16004)
I think this can be achieved by evaluating the formatted string (%U + %S) from time.