[#1569] fix(rust): flaky test for test_ticket_manager (#1570)

What changes were proposed in this pull request?
Use i64 to fix overflow rather than u64

Why are the changes needed?
Fix: #1569. to fix test_ticket_manager

Does this PR introduce any user-facing change?
No.

How was this patch tested?
Existing uts
diff --git a/rust/experimental/server/src/store/mem/ticket.rs b/rust/experimental/server/src/store/mem/ticket.rs
index eaad379..0f69d15 100644
--- a/rust/experimental/server/src/store/mem/ticket.rs
+++ b/rust/experimental/server/src/store/mem/ticket.rs
@@ -47,7 +47,7 @@
     }
 
     pub fn is_timeout(&self, timeout_sec: i64) -> bool {
-        crate::util::current_timestamp_sec() - self.created_time > timeout_sec as u64
+        (crate::util::current_timestamp_sec() - self.created_time) as i64 > timeout_sec
     }
 
     pub fn get_id(&self) -> i64 {