Fix two flakiness sources in test_scrubber_physical_gc_ancestors (#10457)

We currently have some flakiness in
`test_scrubber_physical_gc_ancestors`, see #10391.

The first flakiness kind is about the reconciler not actually becoming
idle within the timeout of 30 seconds. We see continuous forward
progress so this is likely not a hang. We also see this happen in
parallel to a test failure, so is likely due to runners being
overloaded. Therefore, we increase the timeout.

The second flakiness kind is an assertion failure. This one is a little
bit more tricky, but we saw in the successful run that there was some
advance of the lsn between the compaction ran (which created layer
files) and the gc run. Apparently gc rejects reductions to the single
image layer setting if the cutoff lsn is the same as the lsn of the
image layer: it will claim that that layer is newer than the space
cutoff and therefore skip it, while thinking the old layer (that we want
to delete) is the latest one (so it's not deleted).

We address the second flakiness kind by inserting a tiny amount of WAL
between the compaction and gc. This should hopefully fix things.

Related issue: #10391

(not closing it with the merger of the PR as we'll need to validate that
these changes had the intended effect).

Thanks to Chi for going over this together with me in a call.

test_runner/regress/test_storage_scrubber.py

@@ -227,7 +227,9 @@ def test_scrubber_physical_gc_ancestors(neon_env_builder: NeonEnvBuilder, shard_
     new_shard_count = 4
     assert shard_count is None or new_shard_count > shard_count
     shards = env.storage_controller.tenant_shard_split(tenant_id, shard_count=new_shard_count)
-    env.storage_controller.reconcile_until_idle()  # Move shards to their final locations immediately
+    env.storage_controller.reconcile_until_idle(
+        timeout_secs=120
+    )  # Move shards to their final locations immediately
 
     # Create a timeline after split, to ensure scrubber can handle timelines that exist in child shards but not ancestors
     env.storage_controller.pageserver_api().timeline_create(
@@ -269,6 +271,8 @@ def test_scrubber_physical_gc_ancestors(neon_env_builder: NeonEnvBuilder, shard_
         ps.http_client().timeline_compact(
             shard, timeline_id, force_image_layer_creation=True, wait_until_uploaded=True
         )
+        # Add some WAL so that we don't gc at the latest remote consistent lsn
+        workload.churn_rows(1)
         ps.http_client().timeline_gc(shard, timeline_id, 0)
 
     # We will use a min_age_secs=1 threshold for deletion, let it pass