test: fix pthread memory leak

When the thread attributes object returned by pthread_getattr_np() is no
longer required, it should be destroyed using pthread_attr_destroy().

PR-URL: https://github.com/libuv/libuv/pull/2583
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/test-thread.c

@@ -232,6 +232,7 @@ static void thread_check_stack(void* arg) {
   if (expected == 0)
     expected = (size_t)lim.rlim_cur;
   ASSERT(stack_size >= expected);
+  ASSERT(0 == pthread_attr_destroy(&attr));
 #endif
 }