neon_local: set timeline_safekeeper_count if we have less than 3 safekeepers (#12378)

## Problem
- Closes: https://github.com/neondatabase/neon/issues/12298

## Summary of changes
- Set `timeline_safekeeper_count` in `neon_local` if we have less than 3
safekeepers
- Remove `cfg!(feature = "testing")` code from
`safekeepers_for_new_timeline`
- Change `timeline_safekeeper_count` type to `usize`

control_plane/src/local_env.rs

@@ -212,7 +212,7 @@ pub struct NeonStorageControllerConf {
 
     pub use_local_compute_notifications: bool,
 
-    pub timeline_safekeeper_count: Option<i64>,
+    pub timeline_safekeeper_count: Option<usize>,
 
     pub posthog_config: Option<PostHogConfig>,
 

control_plane/src/storage_controller.rs

@@ -638,7 +638,13 @@ impl StorageController {
             args.push("--timelines-onto-safekeepers".to_string());
         }
 
-        if let Some(sk_cnt) = self.config.timeline_safekeeper_count {
+        // neon_local is used in test environments where we often have less than 3 safekeepers.
+        if self.config.timeline_safekeeper_count.is_some() || self.env.safekeepers.len() < 3 {
+            let sk_cnt = self
+                .config
+                .timeline_safekeeper_count
+                .unwrap_or(self.env.safekeepers.len());
+
             args.push(format!("--timeline-safekeeper-count={sk_cnt}"));
         }