tests: prepare test_historic_storage_formats for --timelines-onto-safekeepers (#12214)

## Problem
`test_historic_storage_formats` uses `/tenant_import` to import historic
data. Tenant import does not create timelines onto safekeepers, because
they might already exist on some safekeeper set. If it does, then we may
end up with two different quorums accepting WAL for the same timeline.

If the tenant import is used in a real deployment, the administrator is
responsible for looking for the proper safekeeper set and migrate
timelines into storcon-managed timelines.

- Relates to https://github.com/neondatabase/neon/pull/11712

## Summary of changes
- Create timelines onto safekeepers manually after tenant import in
`test_historic_storage_formats`
- Add a note to tenant import that timelines will be not storcon-managed
after the import.

storage_controller/src/service.rs

@@ -6651,6 +6651,8 @@ impl Service {
 
     /// This is for debug/support only: assuming tenant data is already present in S3, we "create" a
     /// tenant with a very high generation number so that it will see the existing data.
+    /// It does not create timelines on safekeepers, because they might already exist on some
+    /// safekeeper set. So, the timelines are not storcon-managed after the import.
     pub(crate) async fn tenant_import(
         &self,
         tenant_id: TenantId,