stats: fix the data race in test

[alivxxx]

What problem does this PR solve?

Fix the data race:

WARNING: DATA RACE
Read at 0x00c4200dc570 by goroutine 59:
 runtime.mapaccess1_fast32()
     /usr/local/go/src/runtime/hashmap_fast.go:12 +0x0
 github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus.LevelHooks.Fire()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus/hooks.go:27 +0x50
 github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus.Entry.log()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus/entry.go:96 +0x1ba
 github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus.(*Entry).Debug()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus/entry.go:130 +0x110
 github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus.(*Entry).Debugf()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus/entry.go:178 +0x140
 github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus.(*Logger).Debugf()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus/logger.go:118 +0xa7
 github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus.Debugf()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus/exported.go:117 +0x7c
 github.com/pingcap/tidb/ddl.(*worker).start()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/ddl/ddl_worker.go:138 +0x528


Previous write at 0x00c4200dc570 by goroutine 19:
 runtime.mapassign_fast32()
     /usr/local/go/src/runtime/hashmap_fast.go:350 +0x0
 github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus.LevelHooks.Add()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus/hooks.go:20 +0x1a0
 github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus.AddHook()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/vendor/github.com/sirupsen/logrus/exported.go:48 +0xde
 github.com/pingcap/tidb/statistics_test.(*testStatsUpdateSuite).TestLogDetailedInfo()
     /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/statistics/update_test.go:800 +0x59f

What is changed and how it works?

Add the hook at SetUpSuite, so there is no concurrent usage of the hook.

Check List

Tests

• Integration test

Code changes

• Has unexported function/method change

Side effects

• None

Related changes

• None

[alivxxx]

/run-all-tests

[coocood]

LGTM

[alivxxx]

/run-common-test

[zz-jason]

LGTM