etcdserver, wal: Fix tests that were performing unintended casting of…

… int to String.

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)?)

etcdserver/api/v2store/store_test.go

@@ -15,6 +15,7 @@
 package v2store_test
 
 import (
+	"fmt"
 	"testing"
 	"time"
 
@@ -844,7 +845,7 @@ func TestStoreWatchSlowConsumer(t *testing.T) {
 	s.Watch("/foo", true, true, 0) // stream must be true
 	// Fill watch channel with 100 events
 	for i := 1; i <= 100; i++ {
-		s.Set("/foo", false, string(i), v2store.TTLOptionSet{ExpireTime: v2store.Permanent}) // ok
+		s.Set("/foo", false, fmt.Sprint(i), v2store.TTLOptionSet{ExpireTime: v2store.Permanent}) // ok
 	}
 	// testutil.AssertEqual(t, s.WatcherHub.count, int64(1))
 	s.Set("/foo", false, "101", v2store.TTLOptionSet{ExpireTime: v2store.Permanent}) // ok

wal/wal_test.go

@@ -239,7 +239,7 @@ func TestVerify(t *testing.T) {
 
 	// make 5 separate files
 	for i := 0; i < 5; i++ {
-		es := []raftpb.Entry{{Index: uint64(i), Data: []byte("waldata" + string(i+1))}}
+		es := []raftpb.Entry{{Index: uint64(i), Data: []byte(fmt.Sprintf("waldata%d", i+1))}}
 		if err = w.Save(raftpb.HardState{}, es); err != nil {
 			t.Fatal(err)
 		}