test-validator: hold rent constant with --slots-per-epoch (#18318)

(cherry picked from commit 02b14caa5f)

Co-authored-by: Trent Nelson <trent@solana.com>

sdk/program/src/rent.rs

@@ -1,5 +1,6 @@
 #![allow(clippy::integer_arithmetic)]
 //! configuration for network rent
+use crate::clock::DEFAULT_SLOTS_PER_EPOCH;
 
 #[repr(C)]
 #[derive(Serialize, Deserialize, PartialEq, Clone, Copy, Debug, AbiExample)]
@@ -82,6 +83,17 @@ impl Rent {
             ..Rent::default()
         }
     }
+
+    pub fn with_slots_per_epoch(slots_per_epoch: u64) -> Self {
+        let ratio = slots_per_epoch as f64 / DEFAULT_SLOTS_PER_EPOCH as f64;
+        let exemption_threshold = DEFAULT_EXEMPTION_THRESHOLD as f64 * ratio;
+        let lamports_per_byte_year = (DEFAULT_LAMPORTS_PER_BYTE_YEAR as f64 / ratio) as u64;
+        Self {
+            lamports_per_byte_year,
+            exemption_threshold,
+            ..Self::default()
+        }
+    }
 }
 
 #[cfg(test)]