Write unit test(s) for cleanup_expired_logs_for()

# Description

In `checkpoints.rs` the `cleanup_expired_logs_for` function could use at ;least one unit test.  In order to implement it cleanly, I think we should have a `FakeObjectStore` type which we can feed mock `ObjectMeta` structs into to to allow for testing the invocation of the `list` and `delete_stream` invocations that the function makes.

**Related Issue(s)**

* #1763 