tests: refine how compat snapshot is generated (#10342)

## Problem

I noticed in https://github.com/neondatabase/neon/pull/9537 that tests
which work with compat snapshots were writing several hundred MB of
data, which isn't really necessary.

Also, the snapshots are large but don't have the proper variety of
storage format features, e.g. they could just have L0 deltas.

## Summary of changes

- Use smaller scale factor and runtime to generate less data
- Configure a small layer size and use force image layer generation so
that our output contains L1 deltas and image layers, and has a decent
number of entries in the layer map

test_runner/regress/test_compatibility.py

@@ -141,11 +141,18 @@ def test_create_snapshot(
     neon_env_builder.num_safekeepers = 3
     neon_env_builder.enable_pageserver_remote_storage(RemoteStorageKind.LOCAL_FS)
 
-    env = neon_env_builder.init_start()
+    env = neon_env_builder.init_start(
+        initial_tenant_conf={
+            # Miniature layers to enable generating non-trivial layer map without writing lots of data
+            "checkpoint_distance": f"{128 * 1024}",
+            "compaction_threshold": "1",
+            "compaction_target_size": f"{128 * 1024}",
+        }
+    )
     endpoint = env.endpoints.create_start("main")
 
-    pg_bin.run_capture(["pgbench", "--initialize", "--scale=10", endpoint.connstr()])
-    pg_bin.run_capture(["pgbench", "--time=60", "--progress=2", endpoint.connstr()])
+    pg_bin.run_capture(["pgbench", "--initialize", "--scale=1", endpoint.connstr()])
+    pg_bin.run_capture(["pgbench", "--time=30", "--progress=2", endpoint.connstr()])
     pg_bin.run_capture(
         ["pg_dumpall", f"--dbname={endpoint.connstr()}", f"--file={test_output_dir / 'dump.sql'}"]
     )
@@ -157,7 +164,9 @@ def test_create_snapshot(
     pageserver_http = env.pageserver.http_client()
 
     flush_ep_to_pageserver(env, endpoint, tenant_id, timeline_id)
-    pageserver_http.timeline_checkpoint(tenant_id, timeline_id, wait_until_uploaded=True)
+    pageserver_http.timeline_checkpoint(
+        tenant_id, timeline_id, wait_until_uploaded=True, force_image_layer_creation=True
+    )
 
     env.endpoints.stop_all()
     for sk in env.safekeepers: