Interleave benchmark iterations, not just processes

From https://github.com/bytecodealliance/sightglass/issues/138:

> Interleave benchmark runs appropriately. Right now, it looks like the top-level runner does a batch of runs with one engine, then a batch of runs with another. If the runs for different engines/configurations were interleaved at the innermost loop, then system effects that vary over time would at least impact all configurations roughly equally.