scrubber: fix spurious "Missed some shards" errors (#8661)

## Problem

The storage scrubber was reporting warnings for lots of timelines like:
```
WARN Missed some shards at count ShardCount(0) tenant_id=25eb7a83d9a2f90ac0b765b6ca84cf4c
```

These were spurious: these tenants are fine. There was a bug in
accumulating the ShardIndex for each tenant, whereby multiple timelines
would lead us to add the same ShardIndex more than one.

Closes: #8646 

## Summary of changes

- Accumulate ShardIndex in a BTreeSet instead of a Vec
- Extend the test to reproduce the issue

pageserver/src/http/routes.rs

@@ -1787,9 +1787,11 @@ async fn timeline_checkpoint_handler(
         }
 
         if wait_until_uploaded {
+            tracing::info!("Waiting for uploads to complete...");
             timeline.remote_client.wait_completion().await
             // XXX map to correct ApiError for the cases where it's due to shutdown
             .context("wait completion").map_err(ApiError::InternalServerError)?;
+            tracing::info!("Uploads completed up to {}", timeline.get_remote_consistent_lsn_projected().unwrap_or(Lsn(0)));
         }
 
         json_response(StatusCode::OK, ())