tests: add basic coverage for sharding (#6380)

## Problem

The support for sharding in the pageserver was written before
https://github.com/neondatabase/neon/pull/6205 landed, so when it landed
we couldn't directly test sharding.

## Summary of changes

- Add `test_sharding_smoke` which tests the basics of creating a
sharding tenant, creating a timeline within it, checking that data
within it is distributed.
- Add modes to pg_regress tests for running with 4 shards as well as
with 1.

test_runner/fixtures/workload.py

@@ -21,12 +21,21 @@ class Workload:
     - reads, checking we get the right data (`validate`)
     """
 
-    def __init__(self, env: NeonEnv, tenant_id: TenantId, timeline_id: TimelineId):
+    def __init__(
+        self,
+        env: NeonEnv,
+        tenant_id: TenantId,
+        timeline_id: TimelineId,
+        branch_name: Optional[str] = None,
+    ):
         self.env = env
         self.tenant_id = tenant_id
         self.timeline_id = timeline_id
         self.table = "foo"
 
+        # By default, use the default branch name for initial tenant in NeonEnv
+        self.branch_name = branch_name or "main"
+
         self.expect_rows = 0
         self.churn_cursor = 0
 
@@ -35,7 +44,7 @@ class Workload:
     def endpoint(self, pageserver_id: Optional[int] = None) -> Endpoint:
         if self._endpoint is None:
             self._endpoint = self.env.endpoints.create(
-                "main",
+                self.branch_name,
                 tenant_id=self.tenant_id,
                 pageserver_id=pageserver_id,
                 endpoint_id="ep-workload",