migrate to vnodes only in shuffle test

the previous "upgrade to vnodes" here was problematic I think, not
reliably upgrading. and enabling vnodes on an upgraded version before
upgradesstables seems like a bad idea.

watch for log mesg about shuffle "Pausing until token count stabilizes"

this moves that logic into post-upgrade as part of the single upgrade
tests. we'll just use 10 vnodes per node instead of 256. for extra
confirmation that vnode enable and shuffle does no harm, we'll check the
last written counters and data one last time.