fullbench: ensure measured loop lasts a minimum time
fullbench: ensure measured loop lasts a minimum time

to minimize impacts of bad rouding.

While most rounds are supposed to last approximately 1 sec,
the first one is "blindly" evaluated to something short,
which can last < 0.1 sec.

This is a problem for systems with bad accuracy,
since rounding effect will become large.

Ensuring one measure lasts at least BENCHCLOCK_MIN,
it reduces the proportion of maximum possible rounding error.

Cyan4973/FiniteStateEntropy