DATA RACE in the util/topsql/stmtstats.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 0x00c00ddaf980 by goroutine 41122:
runtime.mapassign()
GOROOT/src/runtime/map.go:579 +0x0
github.com/pingcap/tidb/pkg/util/topsql/stmtstats.StatementStatsMap.Merge()
pkg/util/topsql/stmtstats/stmtstats.go:162 +0x390
pkg/server/tests/tests_test.setupForTestTopSQLStatementStats.func1()
pkg/server/tests/tidb_test.go:2032 +0xb2
pkg/server/tests/tests_test.(*mockCollector).CollectStmtStatsMap()
pkg/server/tests/tidb_test.go:1728 +0x42
github.com/pingcap/tidb/pkg/util/topsql/stmtstats.(*aggregator).aggregate.func2()
pkg/util/topsql/stmtstats/aggregator.go:91 +0x54
sync.(*Map).Range()
GOROOT/src/sync/map.go:476 +0x1db
github.com/pingcap/tidb/pkg/util/topsql/stmtstats.(*aggregator).aggregate()
pkg/util/topsql/stmtstats/aggregator.go:90 +0x124
github.com/pingcap/tidb/pkg/util/topsql/stmtstats.(*aggregator).run()
pkg/util/topsql/stmtstats/aggregator.go:71 +0xf1
github.com/pingcap/tidb/pkg/util/topsql/stmtstats.(*aggregator).start.func1()
pkg/util/topsql/stmtstats/aggregator.go:56 +0x33
Previous read at 0x00c00ddaf980 by goroutine 41121:
runtime.mapiternext()
GOROOT/src/runtime/map.go:867 +0x0
pkg/server/tests/tests_test.TestTopSQLStatementStats3()
pkg/server/tests/tidb_test.go:2260 +0x6e4
github.com/pingcap/failpoint.(*Failpoint).Enable()
external/com_github_pingcap_failpoint/failpoint.go:54 +0x3e
github.com/pingcap/failpoint.(*Failpoints).Enable()
external/com_github_pingcap_failpoint/failpoints.go:105 +0x296
github.com/pingcap/failpoint.Enable()
external/com_github_pingcap_failpoint/failpoints.go:222 +0x84
pkg/server/tests/tests_test.TestTopSQLStatementStats3()
pkg/server/tests/tidb_test.go:2223 +0x5f
github.com/pingcap/failpoint.parse()
external/com_github_pingcap_failpoint/terms.go:126 +0xa5
github.com/pingcap/failpoint.newTerms()
external/com_github_pingcap_failpoint/terms.go:98 +0x3e
github.com/pingcap/failpoint.(*Failpoint).Enable()
external/com_github_pingcap_failpoint/failpoint.go:54 +0x3e
github.com/pingcap/failpoint.(*Failpoints).Enable()
external/com_github_pingcap_failpoint/failpoints.go:105 +0x296
github.com/pingcap/failpoint.Enable()
external/com_github_pingcap_failpoint/failpoints.go:222 +0x48a
pkg/server/tests/tests_test.setupForTestTopSQLStatementStats()
pkg/server/tests/tidb_test.go:2050 +0x48b
github.com/pingcap/failpoint.parseTerm()
external/com_github_pingcap_failpoint/terms.go:149 +0x364
github.com/pingcap/failpoint.parse()
external/com_github_pingcap_failpoint/terms.go:126 +0xa5
github.com/pingcap/failpoint.newTerms()
external/com_github_pingcap_failpoint/terms.go:98 +0x3e
github.com/pingcap/failpoint.(*Failpoint).Enable()
external/com_github_pingcap_failpoint/failpoint.go:54 +0x3e
github.com/pingcap/failpoint.(*Failpoints).Enable()
external/com_github_pingcap_failpoint/failpoints.go:105 +0x296
github.com/pingcap/failpoint.Enable()
external/com_github_pingcap_failpoint/failpoints.go:222 +0x444
pkg/server/tests/tests_test.setupForTestTopSQLStatementStats()
pkg/server/tests/tidb_test.go:2049 +0x445
github.com/pingcap/tidb/pkg/domain.(*Domain).rebuildSysVarCache()
pkg/domain/sysvar_cache.go:146 +0x904
github.com/pingcap/tidb/pkg/domain.(*Domain).LoadSysVarCacheLoop()
pkg/domain/domain.go:1722 +0x93
github.com/pingcap/tidb/pkg/session.bootstrapSessionImpl()
pkg/session/session.go:3346 +0x724
github.com/pingcap/tidb/pkg/domain.(*Domain).GetSessionCache()
pkg/domain/sysvar_cache.go:62 +0x50
github.com/pingcap/tidb/pkg/session.(*session).loadCommonGlobalVariablesIfNeeded()
pkg/session/session.go:3723 +0x2ce
github.com/pingcap/tidb/pkg/session.(*session).ExecuteStmt()
pkg/session/session.go:2094 +0x184
github.com/pingcap/tidb/pkg/session.(*session).ExecuteInternal()
pkg/session/session.go:1608 +0x3b9
github.com/pingcap/tidb/pkg/domain.(*Domain).LoadPrivilegeLoop()
pkg/domain/domain.go:1665 +0x108
github.com/pingcap/tidb/pkg/session.bootstrapSessionImpl()
pkg/session/session.go:3339 +0x6ca
github.com/pingcap/tidb/pkg/session.BootstrapSession()
pkg/session/session.go:3260 +0x22a
pkg/server/tests/tests_test.createTidbTestSuiteWithCfg()
pkg/server/tests/tidb_test.go:104 +0x1b5
pkg/server/tests/tests_test.createTidbTestSuite()
pkg/server/tests/tidb_test.go:93 +0x204
pkg/server/tests/tests_test.setupForTestTopSQLStatementStats()
pkg/server/tests/tidb_test.go:2040 +0x296
pkg/server/tests/tests_test.TestTopSQLStatementStats3()
pkg/server/tests/tidb_test.go:2221 +0x3e
testing.tRunner()
GOROOT/src/testing/testing.go:1595 +0x261
testing.(*T).Run.func1()
GOROOT/src/testing/testing.go:1648 +0x44
Goroutine 41122 (running) created at:
github.com/pingcap/tidb/pkg/util/topsql/stmtstats.(*aggregator).start()
pkg/util/topsql/stmtstats/aggregator.go:56 +0x1a4
github.com/pingcap/tidb/pkg/util/topsql/stmtstats.SetupAggregator()
pkg/util/topsql/stmtstats/aggregator.go:146 +0x7a
pkg/server/tests/tests_test.setupForTestTopSQLStatementStats()
pkg/server/tests/tidb_test.go:2023 +0x62
pkg/server/tests/tests_test.TestTopSQLStatementStats3()
pkg/server/tests/tidb_test.go:2221 +0x3e
testing.tRunner()
GOROOT/src/testing/testing.go:1595 +0x261
testing.(*T).Run.func1()
GOROOT/src/testing/testing.go:1648 +0x44
Goroutine 41121 (running) created at:
testing.(*T).Run()
GOROOT/src/testing/testing.go:1648 +0x845
testing.runTests.func1()
GOROOT/src/testing/testing.go:2054 +0x84
testing.tRunner()
GOROOT/src/testing/testing.go:1595 +0x261
testing.runTests()
GOROOT/src/testing/testing.go:2052 +0x8ad
testing.(*M).Run()
GOROOT/src/testing/testing.go:1925 +0xcd7
go.uber.org/goleak.VerifyTestMain()
external/org_uber_go_goleak/testmain.go:53 +0x64
pkg/server/tests/tests_test.TestMain()
pkg/server/tests/main_test.go:73 +0xfcf
main.main()
bazel-out/k8-fastbuild/bin/pkg/server/tests/tests_test_/testmain.go:259 +0x5d1
==================

```

### 4. What is your TiDB version? (Required)