fix preemptive test name (#370)

Author: loongs-zhang

open-coroutine/tests/preemptive.rs

@@ -2,6 +2,6 @@ include!("../examples/preemptive.rs");
 
 #[cfg(not(windows))]
 #[test]
-fn socket_co() -> std::io::Result<()> {
+fn preemptive() -> std::io::Result<()> {
     main()
 }