allocator: unexpected behavior when trying with asim

**Describe the problem**

Motivated by the recent allocator bug, I revisited my intern project and found a
few other cases where the allocator fails to conform to the constraint. They are
all edge cases, and it's unclear whether they share the same underlying root
cause. I explained my observations in the comments for the test cases (I might
be misunderstanding the expected behavior in some cases.)

We fail to move voters/replicas from zone a to zone b to satisfy zone b constraint while keeping region constraints on hold.

Repro using the simulator: https://github.com/cockroachdb/cockroach/commit/a0cc7efea0abe67ac58dddff68f12dd0421de90

Jira issue: CRDB-38061