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/utils.py

@@ -403,7 +403,7 @@ def wait_until(
         try:
             res = func()
         except Exception as e:
-            log.info("waiting for %s iteration %s failed", func, i + 1)
+            log.info("waiting for %s iteration %s failed: %s", func, i + 1, e)
             last_exception = e
             if show_intermediate_error:
                 log.info(e)