roachtest/restore: assert replica distribution

[wenyihu6]

Describe the problem

Currently, restore roachtests do not validate replica distribution. In some runs, the test might pass, but the replica distribution may not be ideal. It might be a good idea to add it.

Jira issue: CRDB-50640

Epic CRDB-50823

[blathers-crl]

Hi @wenyihu6, please add branch-* labels to identify which branch(es) this C-bug affects.

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[blathers-crl]

cc @cockroachdb/disaster-recovery