tests: stabilize test_sharding_split_compaction (#8318)

## Problem

This test incorrectly assumed that a post-split compaction would only
drop content. This was easily destabilized by any changes to image
generation rules.

## Summary of changes

- Before split, do a full image layer generation pass, to guarantee that
post-split compaction should only drop data, never create it.
- Fix the force_image_layer_creation mode of compaction that we use from
tests like this: previously it would try and generate image layers even
if one already existed with the same layer key, which caused compaction
to fail.

test_runner/regress/test_sharding.py

@@ -225,6 +225,12 @@ def test_sharding_split_compaction(neon_env_builder: NeonEnvBuilder, failpoint:
     workload.validate()
     workload.stop()
 
+    # Do a full image layer generation before splitting, so that when we compact after splitting
+    # we should only see sizes decrease (from post-split drops/rewrites), not increase (from image layer generation)
+    env.get_tenant_pageserver(tenant_id).http_client().timeline_compact(
+        tenant_id, timeline_id, force_image_layer_creation=True, wait_until_uploaded=True
+    )
+
     # Split one shard into two
     shards = env.storage_controller.tenant_shard_split(tenant_id, shard_count=2)