DATA RACE at the StatementStatsMap

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

### 2. What did you expect to see? (Required)

### 3. What did you see instead (Required)

```
==================
WARNING: DATA RACE
Write at 0x00c01d78bb30 by goroutine 58814:
runtime.mapassign()
GOROOT/src/runtime/map.go:578 +0x0
github.com/pingcap/tidb/util/topsql/stmtstats.StatementStatsMap.Merge()
util/topsql/stmtstats/stmtstats.go:162 +0x3a9
github.com/pingcap/tidb/server.setupForTestTopSQLStatementStats.func1()
server/tidb_test.go:2033 +0xa4
github.com/pingcap/tidb/server.(*mockCollector).CollectStmtStatsMap()
server/tidb_test.go:1729 +0x45
github.com/pingcap/tidb/util/topsql/stmtstats.(*aggregator).aggregate.func2()
util/topsql/stmtstats/aggregator.go:91 +0x56
sync.(*Map).Range()
GOROOT/src/sync/map.go:476 +0x1db
github.com/pingcap/tidb/util/topsql/stmtstats.(*aggregator).aggregate()
util/topsql/stmtstats/aggregator.go:90 +0xde
github.com/pingcap/tidb/util/topsql/stmtstats.(*aggregator).run()
util/topsql/stmtstats/aggregator.go:71 +0xd1
github.com/pingcap/tidb/util/topsql/stmtstats.(*aggregator).start.func1()
util/topsql/stmtstats/aggregator.go:56 +0x39
Previous read at 0x00c01d78bb30 by goroutine 58813:
runtime.mapiternext()
GOROOT/src/runtime/map.go:866 +0x0
github.com/pingcap/tidb/server.TestTopSQLStatementStats3()
server/tidb_test.go:2261 +0x67d
github.com/pingcap/failpoint.(*Failpoint).Enable()
external/com_github_pingcap_failpoint/failpoint.go:54 +0x44
github.com/pingcap/failpoint.(*Failpoints).Enable()
external/com_github_pingcap_failpoint/failpoints.go:105 +0x276
github.com/pingcap/failpoint.Enable()
external/com_github_pingcap_failpoint/failpoints.go:222 +0x8f
github.com/pingcap/tidb/server.TestTopSQLStatementStats3()
server/tidb_test.go:2224 +0x6b
github.com/pingcap/failpoint.parse()
external/com_github_pingcap_failpoint/terms.go:126 +0xad
github.com/pingcap/failpoint.newTerms()
external/com_github_pingcap_failpoint/terms.go:98 +0x50
github.com/pingcap/failpoint.(*Failpoint).Enable()
external/com_github_pingcap_failpoint/failpoint.go:54 +0x44
github.com/pingcap/failpoint.(*Failpoints).Enable()
external/com_github_pingcap_failpoint/failpoints.go:105 +0x276
github.com/pingcap/failpoint.Enable()
external/com_github_pingcap_failpoint/failpoints.go:222 +0x468
github.com/pingcap/tidb/server.setupForTestTopSQLStatementStats()
server/tidb_test.go:2051 +0x469
github.com/pingcap/failpoint.parseTerm()
external/com_github_pingcap_failpoint/terms.go:149 +0x389
github.com/pingcap/failpoint.parse()
external/com_github_pingcap_failpoint/terms.go:126 +0xad
github.com/pingcap/failpoint.newTerms()
external/com_github_pingcap_failpoint/terms.go:98 +0x50
github.com/pingcap/failpoint.(*Failpoint).Enable()
external/com_github_pingcap_failpoint/failpoint.go:54 +0x44
github.com/pingcap/failpoint.(*Failpoints).Enable()
external/com_github_pingcap_failpoint/failpoints.go:105 +0x276
github.com/pingcap/failpoint.Enable()
external/com_github_pingcap_failpoint/failpoints.go:222 +0x41e
github.com/pingcap/tidb/server.setupForTestTopSQLStatementStats()
server/tidb_test.go:2050 +0x41f
github.com/pingcap/tidb/domain.(*Domain).rebuildSysVarCache()
domain/sysvar_cache.go:146 +0x8c4
github.com/pingcap/tidb/domain.(*Domain).LoadSysVarCacheLoop()
domain/domain.go:1581 +0xa8
github.com/pingcap/tidb/session.BootstrapSession()
session/session.go:3325 +0x6d3
github.com/pingcap/tidb/domain.(*Domain).GetSessionCache()
domain/sysvar_cache.go:62 +0x5c
github.com/pingcap/tidb/session.(*session).loadCommonGlobalVariablesIfNeeded()
session/session.go:3672 +0x104
github.com/pingcap/tidb/session.(*session).ExecuteStmt()
session/session.go:2103 +0x145
github.com/pingcap/tidb/session.(*session).ExecuteInternal()
session/session.go:1638 +0x31b
github.com/pingcap/tidb/domain.(*Domain).LoadPrivilegeLoop()
domain/domain.go:1525 +0x130
github.com/pingcap/tidb/session.BootstrapSession()
session/session.go:3318 +0x684
github.com/pingcap/tidb/server.createTidbTestSuiteWithCfg()
server/tidb_test.go:102 +0x1b6
github.com/pingcap/tidb/server.createTidbTestSuite()
server/tidb_test.go:91 +0x21a
github.com/pingcap/tidb/server.setupForTestTopSQLStatementStats()
server/tidb_test.go:2041 +0x2a4
github.com/pingcap/tidb/server.TestTopSQLStatementStats3()
server/tidb_test.go:2222 +0x4a
testing.tRunner()
GOROOT/src/testing/testing.go:1576 +0x216
testing.(*T).Run.func1()
GOROOT/src/testing/testing.go:1629 +0x47
Goroutine 58814 (running) created at:
github.com/pingcap/tidb/util/topsql/stmtstats.(*aggregator).start()
util/topsql/stmtstats/aggregator.go:56 +0x19a
github.com/pingcap/tidb/util/topsql/stmtstats.SetupAggregator()
util/topsql/stmtstats/aggregator.go:146 +0x86
github.com/pingcap/tidb/server.setupForTestTopSQLStatementStats()
server/tidb_test.go:2024 +0x6e
github.com/pingcap/tidb/server.TestTopSQLStatementStats3()
server/tidb_test.go:2222 +0x4a
testing.tRunner()
GOROOT/src/testing/testing.go:1576 +0x216
testing.(*T).Run.func1()
GOROOT/src/testing/testing.go:1629 +0x47
Goroutine 58813 (running) created at:
testing.(*T).Run()
GOROOT/src/testing/testing.go:1629 +0x805
testing.runTests.func1()
GOROOT/src/testing/testing.go:2036 +0x8d
testing.tRunner()
GOROOT/src/testing/testing.go:1576 +0x216
testing.runTests()
GOROOT/src/testing/testing.go:2034 +0x87c
testing.(*M).Run()
GOROOT/src/testing/testing.go:1906 +0xb44
go.uber.org/goleak.VerifyTestMain()
external/org_uber_go_goleak/testmain.go:53 +0x70
github.com/pingcap/tidb/server.TestMain()
server/main_test.go:74 +0xe76
main.main()
bazel-out/k8-fastbuild/bin/server/server_test_/testmain.go:410 +0x5e8
==================
```
### 4. What is your TiDB version? (Required)

master

HEAD is now at 4878ed3d5 ddl: Coalesce partition (#42476)