storage: Deflake TestReplicaCommandQueueCancellation
storage: Deflake TestReplicaCommandQueueCancellation

Unblocking the first command immediately after unblocking the third
would occasionally cause both channels to be readable in the same select
statement, with non-deterministic results. Wait for the third command to
be canceled before unblocking the first.

Fixes #10502

veteranlu/cockroach,veteranlu/cockroach,glycerine/cockroach,maxlang/cockroach,vaibhavarora/cockroach,glycerine/cockroach,veteranlu/cockroach,glycerine/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,maxlang/cockroach,petermattis/cockroach,maxlang/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,paperstreet/cockroach,maxlang/cockroach,vaibhavarora/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,petermattis/cockroach,glycerine/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,veteranlu/cockroach,glycerine/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,petermattis/cockroach,glycerine/cockroach,petermattis/cockroach,petermattis/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,paperstreet/cockroach,paperstreet/cockroach,petermattis/cockroach,petermattis/cockroach,paperstreet/cockroach,maxlang/cockroach,veteranlu/cockroach,glycerine/cockroach,paperstreet/cockroach,maxlang/cockroach,paperstreet/cockroach