tests: add upload wait in test_scrubber_physical_gc_ancestors (#10260)

## Problem

We see periodic failures in `test_scrubber_physical_gc_ancestors`, where
the logs show that the pageserver is creating image layers that should
cause child shards to no longer reference their parents' layers, but
then the scrubber runs and doesn't find any unreferenced layers.[


https://neon-github-public-dev.s3.amazonaws.com/reports/pr-10256/12582034135/index.html#/testresult/78ea06dea6ba8dd3

From inspecting the code & test, it seems like this could be as simple
as the test failing to wait for uploads before running the scrubber. It
had a 2 second delay built in to satisfy the scrubbers time threshold
checks, which on a lightly loaded machine would also have been easily
enough for uploads to complete, but our test machines are more heavily
loaded all the time.

## Summary of changes

- Wait for uploads to complete after generating images layers in
test_scrubber_physical_gc_ancestors, so that the scrubber should
reliably see the post-compaction metadata.

test_runner/regress/test_storage_scrubber.py

@@ -266,7 +266,9 @@ def test_scrubber_physical_gc_ancestors(neon_env_builder: NeonEnvBuilder, shard_
     for shard in shards:
         ps = env.get_tenant_pageserver(shard)
         assert ps is not None
-        ps.http_client().timeline_compact(shard, timeline_id, force_image_layer_creation=True)
+        ps.http_client().timeline_compact(
+            shard, timeline_id, force_image_layer_creation=True, wait_until_uploaded=True
+        )
         ps.http_client().timeline_gc(shard, timeline_id, 0)
 
     # We will use a min_age_secs=1 threshold for deletion, let it pass