add preemptive test (#369)

Author: loongs-zhang

open-coroutine/tests/preemptive.rs

@@ -0,0 +1,7 @@
+include!("../examples/preemptive.rs");
+
+#[cfg(not(windows))]
+#[test]
+fn socket_co() -> std::io::Result<()> {
+    main()
+}