neon_local: run controller's postgres with fsync=off (#7817)

## Problem

In `test_storage_controller_many_tenants` we
[occasionally](https://neon-github-public-dev.s3.amazonaws.com/reports/main/9155810417/index.html#/testresult/8fbdf57a0e859c2d)
see it hit the retry limit on serializable transactions. That's likely
due to a combination of relative slow fsync on the hetzner nodes running
the test, and the way the test does lots of parallel timeline creations,
putting high load on the drive.

Running the storage controller's db with fsync=off may help here.

## Summary of changes

- Set `fsync=off` in the postgres config for the database used by the
storage controller in tests

control_plane/src/storage_controller.rs

@@ -243,9 +243,13 @@ impl StorageController {
                 anyhow::bail!("initdb failed with status {status}");
             }
 
+            // Write a minimal config file:
+            // - Specify the port, since this is chosen dynamically
+            // - Switch off fsync, since we're running on lightweight test environments and when e.g. scale testing
+            //   the storage controller we don't want a slow local disk to interfere with that.
             tokio::fs::write(
                 &pg_data_path.join("postgresql.conf"),
-                format!("port = {}", self.postgres_port),
+                format!("port = {}\nfsync=off\n", self.postgres_port),
             )
             .await?;
         };