Redundant balance region

[rleungx]

Bug Report

What did you expect to see?

All schedulers work as expected

What did you see instead?

What version of PD are you using (pd-server -V)?

v6.6.0 based serverless version

[rleungx]

This issue barely happens since the test storage volume is small. We are going to add a test case to see if we can improve this case.