Fix potential address-in-use errors in epoll_server test

Wait for all the children to exit before server exits to release all the
resource used by the test.

test/server_epoll/main.c

@@ -9,6 +9,7 @@
 #include <sys/epoll.h>
 #include <sys/socket.h>
 #include <sys/types.h>
+#include <sys/wait.h>
 
 #include "test.h"
 
@@ -142,6 +143,14 @@ int test_ip_socket() {
         }
     }
 
+    // Wait for all the children to exit
+    for (int i = 0; i < proc_num; i++) {
+        if (wait(NULL) < 0) {
+            close_files(2, server_fd, epfd);
+            THROW_ERROR("failed to wait");
+        }
+    }
+
     close_files(2, server_fd, epfd);
     return 0;
 }