test: add test to detect memory leak

The leak itself was fixed in the previous commit.

test/test-leak-1.c

@@ -0,0 +1,38 @@
+#include <errno.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "test-util.h"
+#include "util.h"
+#include "wepoll.h"
+#include "win.h"
+
+int main(void) {
+  HANDLE ephnd;
+  SOCKET sock;
+  struct epoll_event ev;
+  int r;
+
+  ephnd = epoll_create1(0);
+  check(ephnd != NULL);
+
+  sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+  check(sock != INVALID_SOCKET);
+
+  ev.events = 0;
+  ev.data.u64 = 0;
+  r = epoll_ctl(ephnd, EPOLL_CTL_ADD, sock, &ev);
+  check(r == 0);
+
+  r = epoll_wait(ephnd, &ev, 1, 0);
+  check(r == 0);
+
+  r = epoll_ctl(ephnd, EPOLL_CTL_DEL, sock, NULL);
+  check(r == 0);
+
+  r = epoll_close(ephnd);
+  check(r == 0);
+
+  return 0;
+}