storage: fix TestRebalanceInterval race
storage: fix TestRebalanceInterval race

This test was calling `timeutil.SetOffset`, which was racing with calls
to `timeutil.Now` made from `StorePool`'s background goroutine. Now the
test stops the goroutine before making any calls to `timeutil`
functions.

Verified with 200k runs of test race. Was previously easily reproducible
after about 100k runs.

Fixes #6200

paperstreet/cockroach,petermattis/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,paperstreet/cockroach,petermattis/cockroach,sunya123/cockroach,veteranlu/cockroach,sunya123/cockroach,sunya123/cockroach,vivekmenezes/cockroach,glycerine/cockroach,glycerine/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,maxlang/cockroach,paperstreet/cockroach,glycerine/cockroach,cockroachdb/cockroach,sunya123/cockroach,petermattis/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,sunya123/cockroach,arjun4084346/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,arjun4084346/cockroach,sunya123/cockroach,arjun4084346/cockroach,veteranlu/cockroach,glycerine/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,paperstreet/cockroach,glycerine/cockroach,paperstreet/cockroach,arjun4084346/cockroach,maxlang/cockroach,maxlang/cockroach,vivekmenezes/cockroach,petermattis/cockroach,sunya123/cockroach,petermattis/cockroach,veteranlu/cockroach,arjun4084346/cockroach,veteranlu/cockroach,veteranlu/cockroach,cockroachdb/cockroach,glycerine/cockroach,vaibhavarora/cockroach,maxlang/cockroach,petermattis/cockroach,veteranlu/cockroach,maxlang/cockroach,maxlang/cockroach,glycerine/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,cockroachdb/cockroach,petermattis/cockroach,glycerine/cockroach,vaibhavarora/cockroach,maxlang/cockroach,maxlang/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,veteranlu/cockroach,petermattis/cockroach,sunya123/cockroach,arjun4084346/cockroach,cockroachdb/cockroach