storage: disable splits when testing merging last range
storage: disable splits when testing merging last range

This previously pitted table splits against the attempt to merge the
last range in the system. Disabling table splits fixes #3928.

marcuswestin/cockroach,arjun4084346/cockroach,paperstreet/cockroach,arjun4084346/cockroach,marcuswestin/cockroach,marcuswestin/cockroach,marcuswestin/cockroach,vivekmenezes/cockroach,petermattis/cockroach,bg451/cockroach,maxlang/cockroach,nieyy/cockroach,il9ue/cockroach,veteranlu/cockroach,petermattis/cockroach,arjun4084346/cockroach,sunya123/cockroach,vivekmenezes/cockroach,cuongdo/cockroach,glycerine/cockroach,arjun4084346/cockroach,nieyy/cockroach,veteranlu/cockroach,cockroachdb/cockroach,petermattis/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,glycerine/cockroach,nieyy/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,cuongdo/cockroach,vivekmenezes/cockroach,il9ue/cockroach,maxlang/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,nieyy/cockroach,nieyy/cockroach,cuongdo/cockroach,cuongdo/cockroach,cockroachdb/cockroach,veteranlu/cockroach,veteranlu/cockroach,sunya123/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,il9ue/cockroach,sunya123/cockroach,marcuswestin/cockroach,arjun4084346/cockroach,petermattis/cockroach,sunya123/cockroach,petermattis/cockroach,nieyy/cockroach,vaibhavarora/cockroach,maxlang/cockroach,bg451/cockroach,glycerine/cockroach,cockroachdb/cockroach,paperstreet/cockroach,cuongdo/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,bg451/cockroach,maxlang/cockroach,paperstreet/cockroach,veteranlu/cockroach,maxlang/cockroach,glycerine/cockroach,paperstreet/cockroach,sunya123/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,nieyy/cockroach,sunya123/cockroach,bg451/cockroach,marcuswestin/cockroach,sunya123/cockroach,il9ue/cockroach,glycerine/cockroach,glycerine/cockroach,vivekmenezes/cockroach,il9ue/cockroach,nieyy/cockroach,veteranlu/cockroach,cuongdo/cockroach,maxlang/cockroach,bg451/cockroach,il9ue/cockroach,petermattis/cockroach,glycerine/cockroach,bg451/cockroach,arjun4084346/cockroach,marcuswestin/cockroach,petermattis/cockroach,il9ue/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,cuongdo/cockroach,marcuswestin/cockroach,sunya123/cockroach,glycerine/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,petermattis/cockroach,paperstreet/cockroach,arjun4084346/cockroach,il9ue/cockroach,vaibhavarora/cockroach,cuongdo/cockroach,paperstreet/cockroach,bg451/cockroach,cockroachdb/cockroach