data race even for the example_test.go

[maxshuang]

================== WARNING: DATA RACE Read at 0x00c0006b29a8 by goroutine 95: github.com/pingcap/tidb/server.(*Server).Close() /root/go/pkg/mod/github.com/pingcap/[email protected]/server/server.go:415 +0x11e github.com/WangXiangUSTC/tidb-lite.(*TiDBServer).serverShutdown() /root/tidb-lite/tidb.go:303 +0xb0 github.com/WangXiangUSTC/tidb-lite.(*TiDBServer).Close() /root/tidb-lite/tidb.go:169 +0x847 github.com/WangXiangUSTC/tidb-lite/example.(*testExampleSuite).TestGetRowCount() /root/tidb-lite/example/example_test.go:55 +0x83