TestLoadLargeRules is flaky

## Flaky Test
### Which jobs are failing
```text
{"level":"warn","ts":"2026-03-12T15:52:30.525421+0800","caller":"etcdserver/util.go:170","msg":"apply request took too long","took":"124.859275ms","expected-duration":"100ms","prefix":"","request":"header: put:","response":"size:5"}
{"level":"info","ts":"2026-03-12T15:52:30.525721+0800","caller":"traceutil/trace.go:171","msg":"trace[490458128] put","detail":"{key:/pd/0/region_label/test_4844; req_size:267; response_revision:4845; }","duration":"147.304769ms","start":"2026-03-12T15:52:30.378332+0800","end":"2026-03-12T15:52:30.525637+0800","steps":["trace[490458128] 'process raft request' (duration: 22.132178ms)","trace[490458128] 'marshal mvccpb.KeyValue' (duration: 124.398704ms)"],"step_count":2}
{"level":"info","ts":"2026-03-12T15:52:30.731723+0800","caller":"traceutil/trace.go:171","msg":"trace[2075406135] put","detail":"{key:/pd/0/region_label/test_4845; req_size:267; response_revision:4846; }","duration":"154.89274ms","start":"2026-03-12T15:52:30.576706+0800","end":"2026-03-12T15:52:30.731598+0800","steps":["trace[2075406135] 'process raft request' (duration: 132.16049ms)","trace[2075406135] 'marshal mvccpb.KeyValue' (duration: 22.41421ms)"],"step_count":2}
panic: test timed out after 5m0s
running tests:
TestLoadLargeRules (5m0s)
goroutine 6025 [running]:
testing.(*M).startAlarm.func1()
/usr/local/go/src/testing/testing.go:2682 +0x605
created by time.goFunc
/usr/local/go/src/time/sleep.go:215 +0x45
goroutine 1 [chan receive, 5 minutes]:
```
### CI link
https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/tikv_pd/10339/pull-unit-test-next-gen-1/2031997832684113920
### Reason for failure (if possible)

### Anything else
test=TestLoadLargeRules
package=github.com/tikv/pd/pkg/mcs/scheduling/server/rule
signatures=TIMEOUT_PANIC,PANIC