fix tests

tests/stack_overflow.rs

@@ -49,7 +49,7 @@ use xe_os::{exit_qemu, serial_println, QemuExitCode};
 extern "x86-interrupt" fn test_double_fault_handler(
     _stack_frame: &mut InterruptStackFrame,
     _error_code: u64,
-) {
+) -> ! {
     serial_println!("[ok]");
     exit_qemu(QemuExitCode::Success);
     loop {}