test_sharding_ingress: bigger data, skip in debug mode (#6859)

## Problem

Accidentally merged #6852 without this test stability change. The test
as-written could sometimes fail on debug-pg14.

## Summary of changes

- Write more data so that the test can more reliably assert on the ratio
of total layers to small layers
- Skip the test in debug mode, since writing any more than a tiny bit of
data tends to result in a flaky test in the much slower debug
environment.

test_runner/regress/test_sharding.py

@@ -1,3 +1,6 @@
+import os
+
+import pytest
 from fixtures.log_helper import log
 from fixtures.neon_fixtures import (
     NeonEnvBuilder,
@@ -286,6 +289,12 @@ def test_sharding_split_smoke(
     env.attachment_service.consistency_check()
 
 
+@pytest.mark.skipif(
+    # The quantity of data isn't huge, but debug can be _very_ slow, and the things we're
+    # validating in this test don't benefit much from debug assertions.
+    os.getenv("BUILD_TYPE") == "debug",
+    reason="Avoid running bulkier ingest tests in debug mode",
+)
 def test_sharding_ingest(
     neon_env_builder: NeonEnvBuilder,
 ):
@@ -319,10 +328,10 @@ def test_sharding_ingest(
 
     workload = Workload(env, tenant_id, timeline_id)
     workload.init()
-    workload.write_rows(512, upload=False)
-    workload.write_rows(512, upload=False)
-    workload.write_rows(512, upload=False)
-    workload.write_rows(512, upload=False)
+    workload.write_rows(4096, upload=False)
+    workload.write_rows(4096, upload=False)
+    workload.write_rows(4096, upload=False)
+    workload.write_rows(4096, upload=False)
     workload.validate()
 
     small_layer_count = 0
@@ -361,7 +370,12 @@ def test_sharding_ingest(
     # - Because we roll layers on checkpoint_distance * shard_count, we expect to obey the target
     #   layer size on average, but it is still possible to write some tiny layers.
     log.info(f"Totals: {small_layer_count} small layers, {ok_layer_count} ok layers")
-    assert float(small_layer_count) / float(ok_layer_count) < 0.25
+    if small_layer_count <= shard_count:
+        # If each shard has <= 1 small layer
+        pass
+    else:
+        # General case:
+        assert float(small_layer_count) / float(ok_layer_count) < 0.25
 
     # Each shard may emit up to one huge layer, because initdb ingest doesn't respect checkpoint_distance.
     assert huge_layer_count <= shard_count