kbfs_ops_test: fix TestKBFSOpsRecentHistorySync flake
kbfs_ops_test: fix TestKBFSOpsRecentHistorySync flake

For partial syncs based on recent edit activity, there's a race
between the MD update getting delivered and the edit history being
processed.

keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client