tests: deflake test_sharding_split_unsharded (#6699)

## Problem

This test was a subset of the larger sharding test, and it missed the
validate() call on workload that was implicitly waiting for a tenant to
become active before trying to split it. It could therefore fail to
split due to tenant not yet being active.

## Summary of changes

- Insert .validate() call, and move the Workload setup to after the
check of shard ID (as the shard ID check should pass immediately)

test_runner/regress/test_sharding.py

@@ -95,14 +95,15 @@ def test_sharding_split_unsharded(
     tenant_id = env.initial_tenant
     timeline_id = env.initial_timeline
 
-    workload = Workload(env, tenant_id, timeline_id, branch_name="main")
-    workload.init()
-    workload.write_rows(256)
-
     # Check that we created with an unsharded TenantShardId: this is the default,
     # but check it in case we change the default in future
     assert env.attachment_service.inspect(TenantShardId(tenant_id, 0, 0)) is not None
 
+    workload = Workload(env, tenant_id, timeline_id, branch_name="main")
+    workload.init()
+    workload.write_rows(256)
+    workload.validate()
+
     # Split one shard into two
     env.attachment_service.tenant_shard_split(tenant_id, shard_count=2)