Data race detected when running TestDDLTableCreateBackfillTable

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

See pipeline execution: https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftidb%2Fghpr_unit_test/detail/ghpr_unit_test/35371/pipeline/

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

No error

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

```
WARNING: DATA RACE

Read at 0x00c00100a858 by goroutine 78732:

github.com/dgraph-io/ristretto.(*Cache).Get()
external/com_github_dgraph_io_ristretto/cache.go:225 +0x47
github.com/pingcap/tidb/pkg/store/copr.(*coprCache).Get()
pkg/store/copr/coprocessor_cache.go:168 +0xa4
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).buildCacheKey()
pkg/store/copr/coprocessor.go:1640 +0x410
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).handleTaskOnce()
pkg/store/copr/coprocessor.go:1170 +0x4ea
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).handleTask()
pkg/store/copr/coprocessor.go:1130 +0x210
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).run()
pkg/store/copr/coprocessor.go:817 +0x1ce
github.com/pingcap/tidb/pkg/store/copr.(*copIterator).open.func1()
pkg/store/copr/coprocessor.go:861 +0x4f

Previous write at 0x00c00100a858 by goroutine 94:
github.com/dgraph-io/ristretto.(*Cache).Close()
external/com_github_dgraph_io_ristretto/cache.go:363 +0xf2
github.com/pingcap/tidb/pkg/store/copr.(*Store).Close()
pkg/store/copr/store.go:103 +0x96
github.com/pingcap/tidb/pkg/store/mockstore/mockstorage.(*mockStorage).Close()
pkg/store/mockstore/mockstorage/storage.go:120 +0xe4
github.com/pingcap/tidb/pkg/session.TestDDLTableCreateBackfillTable.func1()
pkg/session/bootstrap_test.go:227 +0x42
runtime.gopanic()
GOROOT/src/runtime/panic.go:920 +0x26f
github.com/pingcap/tidb/pkg/session.(*session).ExecuteStmt()
pkg/session/session.go:2204 +0xdc5
github.com/pingcap/tidb/pkg/session.(*session).Execute()
pkg/session/session.go:1670 +0x356
github.com/pingcap/tidb/pkg/session.exec()
pkg/session/testutil.go:80 +0xc8
github.com/pingcap/tidb/pkg/session.MustExec()
pkg/session/testutil.go:63 +0x98
github.com/pingcap/tidb/pkg/session.TestDDLTableCreateBackfillTable()
pkg/session/bootstrap_test.go:239 +0x1eb
github.com/pingcap/tidb/pkg/domain.(*Domain).rebuildSysVarCache()
pkg/domain/sysvar_cache.go:146 +0x7d0
github.com/pingcap/tidb/pkg/domain.(*Domain).LoadSysVarCacheLoop()
pkg/domain/domain.go:1686 +0xa4
github.com/pingcap/tidb/pkg/session.bootstrapSessionImpl()
pkg/session/session.go:3368 +0x7aa
github.com/pingcap/tidb/pkg/domain.(*Domain).GetSessionCache()
pkg/domain/sysvar_cache.go:62 +0x64
github.com/pingcap/tidb/pkg/session.(*session).loadCommonGlobalVariablesIfNeeded()
pkg/session/session.go:3740 +0x130
github.com/pingcap/tidb/pkg/session.(*session).ExecuteStmt()
pkg/session/session.go:2132 +0x15a
github.com/pingcap/tidb/pkg/session.(*session).ExecuteInternal()
pkg/session/session.go:1645 +0x35a
github.com/pingcap/tidb/pkg/domain.(*Domain).LoadPrivilegeLoop()
pkg/domain/domain.go:1629 +0x11a
github.com/pingcap/tidb/pkg/session.bootstrapSessionImpl()
pkg/session/session.go:3361 +0x74a
github.com/pingcap/tidb/pkg/session.BootstrapSession()
pkg/session/session.go:3282 +0x4b
github.com/pingcap/tidb/pkg/session.CreateStoreAndBootstrap()
pkg/session/testutil.go:45 +0x78
github.com/pingcap/tidb/pkg/session.TestDDLTableCreateBackfillTable()
pkg/session/bootstrap_test.go:226 +0x3b
testing.tRunner()
GOROOT/src/testing/testing.go:1595 +0x238
testing.(*T).Run.func1()
GOROOT/src/testing/testing.go:1648 +0x44

Goroutine 78732 (running) created at:

github.com/pingcap/tidb/pkg/store/copr.(*copIterator).open()
pkg/store/copr/coprocessor.go:861 +0x125
github.com/pingcap/tidb/pkg/store/copr.(*CopClient).Send()
pkg/store/copr/coprocessor.go:105 +0x3e9
github.com/pingcap/tidb/pkg/distsql.Select()
pkg/distsql/distsql.go:91 +0xa45
github.com/pingcap/tidb/pkg/distsql.SelectWithRuntimeStats()
pkg/distsql/distsql.go:146 +0xb7
github.com/pingcap/tidb/pkg/executor.selectResultHook.SelectResult()
pkg/executor/table_reader.go:62 +0x1d7
github.com/pingcap/tidb/pkg/executor.(*TableReaderExecutor).buildResp()
pkg/executor/table_reader.go:337 +0x904
github.com/pingcap/tidb/pkg/executor.(*TableReaderExecutor).Open()
pkg/executor/table_reader.go:218 +0x1156
github.com/pingcap/tidb/pkg/executor.(*ExecStmt).openExecutor()
pkg/executor/adapter.go:1216 +0x101
github.com/pingcap/tidb/pkg/executor.(*ExecStmt).Exec()
pkg/executor/adapter.go:544 +0xfe7
github.com/pingcap/tidb/pkg/session.runStmt()
pkg/session/session.go:2421 +0x701
github.com/pingcap/tidb/pkg/session.(*session).ExecuteStmt()
pkg/session/session.go:2271 +0x1bc4
github.com/pingcap/tidb/pkg/session.(*session).ExecuteInternalStmt()
pkg/session/session.go:2122 +0x1ca
github.com/pingcap/tidb/pkg/session.(*session).ExecRestrictedSQL.func1()
pkg/session/session.go:2087 +0x38c
github.com/pingcap/tidb/pkg/session.(*session).withRestrictedSQLExecutor()
pkg/session/session.go:2073 +0x3a5
github.com/pingcap/tidb/pkg/session.(*session).ExecRestrictedSQL()
pkg/session/session.go:2077 +0x129
github.com/pingcap/tidb/pkg/domain.(*Domain).refreshMDLCheckTableInfo()
pkg/domain/domain.go:774 +0x477
github.com/pingcap/tidb/pkg/domain.(*Domain).loadSchemaInLoop()
pkg/domain/domain.go:927 +0x988
github.com/pingcap/tidb/pkg/domain.(*Domain).Init.func2()
pkg/domain/domain.go:1256 +0x65
github.com/pingcap/tidb/pkg/util.(*WaitGroupEnhancedWrapper).Run.func1()
pkg/util/wait_group_wrapper.go:96 +0xd0

Goroutine 94 (running) created at:
testing.(*T).Run()
GOROOT/src/testing/testing.go:1648 +0x82a
testing.runTests.func1()
GOROOT/src/testing/testing.go:2054 +0x84
testing.tRunner()
GOROOT/src/testing/testing.go:1595 +0x238
testing.runTests()
GOROOT/src/testing/testing.go:2052 +0x896
testing.(*M).Run()
GOROOT/src/testing/testing.go:1925 +0xb57
github.com/pingcap/tidb/pkg/testkit/testmain.(*testingM).Run()
pkg/testkit/testmain/wrapper.go:27 +0x61
go.uber.org/goleak.VerifyTestMain()
external/org_uber_go_goleak/testmain.go:53 +0x64
github.com/pingcap/tidb/pkg/session.TestMain()
pkg/session/main_test.go:66 +0xd77
main.main()
bazel-out/k8-fastbuild/bin/pkg/session/session_test_/testmain.go:295 +0x78e
==================
```

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