etcdserver: add failpoints walBeforeSync and walAfterSync

Signed-off-by: Benjamin Wang <wachao@vmware.com>

server/wal/wal.go

@@ -939,7 +939,10 @@ func (w *WAL) Save(st raftpb.HardState, ents []raftpb.Entry) error {
 	}
 	if curOff < SegmentSizeBytes {
 		if mustSync {
-			return w.sync()
+			// gofail: var walBeforeSync struct{}
+			err = w.sync()
+			// gofail: var walAfterSync struct{}
+			return err
 		}
 		return nil
 	}