Unstable test TestMultiSchemaPartitionByGlobalIndex

## 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)
```
multi_domain_test.go:996:
Error Trace: pkg/ddl/tests/partition/multi_domain_test.go:996
pkg/ddl/tests/partition/multi_domain_test.go:827
pkg/ddl/tests/partition/multi_domain_test.go:577
Error: Should be false
Test: TestMultiSchemaPartitionByGlobalIndex
Messages: Old table id 114 has still entries!
--- FAIL: TestMultiSchemaPartitionByGlobalIndex (2.86s)
```
https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftidb%2Fghpr_unit_test/detail/ghpr_unit_test/36587/pipeline/

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

The failure is in pkg/ddl/tests/partition/multi_domain_test.go, with the assertion at line 996 and callers at lines 827 and 577. Start by running TestMultiSchemaPartitionByGlobalIndex and inspect the linked Jenkins run. Done means the test no longer flakes and the old table has no remaining entries.

Written by the indexing model from the issue text.