backupccl: better test range bound export requests

https://github.com/cockroachdb/cockroach/pull/114268 split backup export request spans to be bound by existing ranges. We ought to better test this behavior in a unit test. Here's one way to do this:

1. Begin a backup of an empty table, halt it just after distsql planning via a testing hook
2. Split the table into 100 ranges
3. Ensure if `bulkio.backup.presplit_request_spans.enabled` is true, we send around 100 export requests.

Jira issue: CRDB-34109