If the `wg.Add(1)` is called inside a goroutine, there is no guarantee
that `wg.Wait` is called before all `wg.Add(1)`. So `wg.Add(1)` should
be called before goroutine starts. Otherwise, the following error
occurs:
```
panic: sync: WaitGroup is reused before previous Wait has returned [recovered]
panic: sync: WaitGroup is reused before previous Wait has returned
goroutine 18 [running]:
testing.tRunner.func1.2(0x51cac0, 0x56ebb0)
/usr/local/go/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc00009e300)
/usr/local/go/src/testing/testing.go:1146 +0x4b6
panic(0x51cac0, 0x56ebb0)
/usr/local/go/src/runtime/panic.go:965 +0x1b9
sync.(*WaitGroup).Wait(0xc000098080)
/usr/local/go/src/sync/waitgroup.go:132 +0xae
github.com/cubefs/cubefs/util/synclist.TestSyncPushBach(0xc00009e300)
/root/tmp/cfs/go/src/github.com/cubefs/cubefs/util/synclist/synclist_test.go:79 +0xf3
testing.tRunner(0xc00009e300, 0x54f988)
/usr/local/go/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
/usr/local/go/src/testing/testing.go:1238 +0x2b3
```
Signed-off-by: Sheng Yong <shengyong2021@gmail.com>
