flaky test TestCursorFetchShouldSpill

## 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)

```
conn_stmt_test.go:288:
Error Trace: pkg/server/conn_stmt_test.go:288
Error: Received unexpected error:
[executor:8175]Your query has been cancelled due to exceeding the allowed memory limit for a single SQL query. Please try narrowing your query scope or increase the tidb_mem_quota_query limit and try again.[conn=6936687692538800719]
github.com/pingcap/errors.AddStack
external/com_github_pingcap_errors/errors.go:174
github.com/pingcap/errors.(*Error).GenWithStackByArgs
external/com_github_pingcap_errors/normalize.go:164
github.com/pingcap/tidb/pkg/util/sqlkiller.(*SQLKiller).HandleSignal
pkg/util/sqlkiller/sqlkiller.go:56
github.com/pingcap/tidb/pkg/util/memory.(*PanicOnExceed).Action
pkg/util/memory/action.go:172
github.com/pingcap/tidb/pkg/util/memory.(*Tracker).Consume.func1
pkg/util/memory/tracker.go:437
github.com/pingcap/tidb/pkg/util/memory.(*Tracker).Consume
pkg/util/memory/tracker.go:464
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).sendToRespCh
pkg/store/copr/coprocessor.go:1001
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).handleCopResponse
pkg/store/copr/coprocessor.go:1442
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).handleCopPagingResult
pkg/store/copr/coprocessor.go:1342
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).handleTaskOnce
pkg/store/copr/coprocessor.go:1275
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).handleTask
pkg/store/copr/coprocessor.go:1130
github.com/pingcap/tidb/pkg/store/copr.(*copIteratorWorker).run
pkg/store/copr/coprocessor.go:817
runtime.goexit
src/runtime/asm_amd64.s:1650
Test: TestCursorFetchShouldSpill
```

https://tiprow.hawkingrei.com/view/gs/pingcapprow/pr-logs/pull/pingcap_tidb/48430/tiprow_fast_test/1722543643119587328#1:build-log.txt%3A1407

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