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`

storage_controller/src/service/safekeeper_service.rs

@@ -1,4 +1,3 @@
-use std::cmp::max;
 use std::collections::HashSet;
 use std::str::FromStr;
 use std::sync::Arc;
@@ -654,13 +653,7 @@ impl Service {
             )
         });
         // Number of safekeepers in different AZs we are looking for
-        let mut wanted_count = self.config.timeline_safekeeper_count as usize;
-        // TODO(diko): remove this when `timeline_safekeeper_count` option is in the release
-        // branch and is specified in tests/neon_local config.
-        if cfg!(feature = "testing") && all_safekeepers.len() < wanted_count {
-            // In testing mode, we can have less safekeepers than the config says
-            wanted_count = max(all_safekeepers.len(), 1);
-        }
+        let wanted_count = self.config.timeline_safekeeper_count;
 
         let mut sks = Vec::new();
         let mut azs = HashSet::new();