br restore splits too much regions if there are too many small tables whose data size is less than region size.

[Leavrth]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. create so many tables, but each table only has a little data.
2. backup cluster with split-table=false/split-region-on-table=false
3. restore cluster

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

the number of regions of downstream cluster is closed to the number of upstream cluster.

3. What did you see instead (Required)

the number of regions has many regions.

4. What is your TiDB version? (Required)

master

[BornChanger]

It's an extension of #50701

[Leavrth]

We need to do some clean work at first. See #52877