ddl: flaky TiFlash replica test `TestInfoSchemaForTiFlashReplica` & `TestSetTiFlashReplicaForTemporaryTable`

[bb7133]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Run the test cases(on my local Macbook laptop) like:

make failpoint-enable && (pushd pkg/ddl >/dev/null; go test -run 'TestInfoSchemaForTiFlashReplica|TestSetTiFlashReplicaForTemporaryTable' --tags=intest -count=1; rc=$?; popd >/dev/null;
make failpoint-disable; exit $rc)

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

The test passes.

3. What did you see instead (Required)

The test failed - 'getting empty result' after a timeout.

4. What is your TiDB version? (Required)

The latest main branch(2f9776e)