Fix intermittent validator_exit test failure (#23594)

* run validator_exit_test sequentially

* limit validator exit run to its own serial run subset
add 10ms delay in the validator exit tests

* fix intermittent validator exit failure

* no sleep

* undo the code move

core/src/validator.rs

@@ -1800,7 +1800,6 @@ mod tests {
         std::fs::remove_dir_all,
     };
 
-    #[test]
     fn validator_exit() {
         solana_logger::setup();
         let leader_keypair = Keypair::new();
@@ -1880,7 +1879,6 @@ mod tests {
         }
     }
 
-    #[test]
     fn validator_parallel_exit() {
         let leader_keypair = Keypair::new();
         let leader_node = Node::new_localhost_with_pubkey(&leader_keypair.pubkey());
@@ -1928,6 +1926,12 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_validator_exit() {
+        validator_exit();
+        validator_parallel_exit();
+    }
+
     #[test]
     fn test_wait_for_supermajority() {
         solana_logger::setup();