Fixes following problems during "./etcd# go test ./..."
> go.etcd.io/etcd/v3/etcdserver/api/v2store_test
etcdserver/api/v2store/store_test.go:847:24: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
> go.etcd.io/etcd/v3/wal
wal/wal_test.go:242:68: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)