Do not exit after testcases in main

src/main.rs

@@ -4,14 +4,18 @@
 #[macro_use]
 extern crate log;
 extern crate alloc;
-extern crate axstd;
+#[macro_use]
+extern crate axstd as std;
 
 mod loader;
 mod mm;
 mod syscall;
 mod task;
 use alloc::sync::Arc;
 
+use std::thread;
+use std::time::Duration;
+
 use axhal::arch::UspaceContext;
 use axsync::Mutex;
 
@@ -34,4 +38,9 @@ fn main() {
         let exit_code = user_task.join();
         info!("User task {} exited with code: {:?}", testcase, exit_code);
     }
+
+    loop {
+        println!("Starry tick...");
+        thread::sleep(Duration::from_secs(1));
+    }
 }