tests: add infra and test for storcon leadership transfer (#8587)

## Problem
https://github.com/neondatabase/neon/pull/8588 implemented the mechanism
for storage controller
leadership transfers. However, there's no tests that exercise the
behaviour.

## Summary of changes
1. Teach `neon_local` how to handle multiple storage controller
instances. Each storage controller
instance gets its own subdirectory (`storage_controller_1, ...`).
`storage_controller start|stop` subcommands
have also been extended to optionally accept an instance id.
2. Add a storage controller proxy test fixture. It's a basic HTTP server
that forwards requests from pageserver
and test env to the currently configured storage controller.
3. Add a test which exercises storage controller leadership transfer.
4. Finally fix a couple bugs that the test surfaced

test_runner/fixtures/storage_controller_proxy.py

@@ -0,0 +1,73 @@
+import re
+from typing import Any, Optional
+
+import pytest
+import requests
+from pytest_httpserver import HTTPServer
+from werkzeug.datastructures import Headers
+from werkzeug.wrappers.request import Request
+from werkzeug.wrappers.response import Response
+
+from fixtures.log_helper import log
+
+
+class StorageControllerProxy:
+    def __init__(self, server: HTTPServer):
+        self.server: HTTPServer = server
+        self.listen: str = f"http://{server.host}:{server.port}"
+        self.routing_to: Optional[str] = None
+
+    def route_to(self, storage_controller_api: str):
+        self.routing_to = storage_controller_api
+
+    def port(self) -> int:
+        return self.server.port
+
+    def upcall_api_endpoint(self) -> str:
+        return f"{self.listen}/upcall/v1"
+
+
+def proxy_request(method: str, url: str, **kwargs) -> requests.Response:
+    return requests.request(method, url, **kwargs)
+
+
+@pytest.fixture(scope="function")
+def storage_controller_proxy(make_httpserver):
+    """
+    Proxies requests into the storage controller to the currently
+    selected storage controller instance via `StorageControllerProxy.route_to`.
+
+    This fixture is intended for tests that need to run multiple instances
+    of the storage controller at the same time.
+    """
+    server = make_httpserver
+
+    self = StorageControllerProxy(server)
+
+    log.info(f"Storage controller proxy listening on {self.listen}")
+
+    def handler(request: Request):
+        if self.route_to is None:
+            log.info(f"Storage controller proxy has no routing configured for {request.url}")
+            return Response("Routing not configured", status=503)
+
+        route_to_url = f"{self.routing_to}{request.path}"
+
+        log.info(f"Routing {request.url} to {route_to_url}")
+
+        args: dict[str, Any] = {"headers": request.headers}
+        if request.is_json:
+            args["json"] = request.json
+
+        response = proxy_request(request.method, route_to_url, **args)
+
+        headers = Headers()
+        for key, value in response.headers.items():
+            headers.add(key, value)
+
+        return Response(response.content, headers=headers, status=response.status_code)
+
+    self.server.expect_request(re.compile(".*")).respond_with_handler(handler)
+
+    yield self
+    server.clear()