tests: use fewer pageservers in test_sharding_split_smoke (#9804)

## Problem

This test uses a gratuitous number of pageservers (16). This works fine
when there are plenty of system resources, but causes issues on test
runners that have limited resources and run many tests concurrently.

Related: #9802

## Summary of changes

- Split from 2 shards to 4, instead of 4 to 8
- Don't give every shard a separate pageserver, let two locations share
each pageserver.

Net result is 4 pageservers instead of 16

test_runner/regress/test_sharding.py

@@ -515,11 +515,12 @@ def test_sharding_split_smoke(
 
     """
 
-    # We will start with 4 shards and split into 8, then migrate all those
-    # 8 shards onto separate pageservers
-    shard_count = 4
-    split_shard_count = 8
-    neon_env_builder.num_pageservers = split_shard_count * 2
+    # Shard count we start with
+    shard_count = 2
+    # Shard count we split into
+    split_shard_count = 4
+    # We will have 2 shards per pageserver once done (including secondaries)
+    neon_env_builder.num_pageservers = split_shard_count
 
     # 1MiB stripes: enable getting some meaningful data distribution without
     # writing large quantities of data in this test.  The stripe size is given
@@ -591,7 +592,7 @@ def test_sharding_split_smoke(
 
     workload.validate()
 
-    assert len(pre_split_pageserver_ids) == 4
+    assert len(pre_split_pageserver_ids) == shard_count
 
     def shards_on_disk(shard_ids):
         for pageserver in env.pageservers:
@@ -654,9 +655,9 @@ def shards_on_disk(shard_ids):
     # - shard_count reconciles for the original setup of the tenant
     # - shard_count reconciles for detaching the original secondary locations during split
     # - split_shard_count reconciles during shard splitting, for setting up secondaries.
-    # - shard_count of the child shards will need to fail over to their secondaries
-    # - shard_count of the child shard secondary locations will get moved to emptier nodes
-    expect_reconciles = shard_count * 2 + split_shard_count + shard_count * 2
+    # - split_shard_count/2 of the child shards will need to fail over to their secondaries (since we have 8 shards and 4 pageservers, only 4 will move)
+    expect_reconciles = shard_count * 2 + split_shard_count + split_shard_count / 2
+
     reconcile_ok = env.storage_controller.get_metric_value(
         "storage_controller_reconcile_complete_total", filter={"status": "ok"}
     )
@@ -720,22 +721,10 @@ def check_effective_tenant_config():
     # dominated by shard count.
     log.info(f"total: {total}")
     assert total == {
-        1: 1,
-        2: 1,
-        3: 1,
-        4: 1,
-        5: 1,
-        6: 1,
-        7: 1,
-        8: 1,
-        9: 1,
-        10: 1,
-        11: 1,
-        12: 1,
-        13: 1,
-        14: 1,
-        15: 1,
-        16: 1,
+        1: 2,
+        2: 2,
+        3: 2,
+        4: 2,
     }
 
     # The controller is not required to lay out the attached locations in any particular way, but