Skip test pending libmongoc segfault fix (CDRIVER-4290)

Author: jmikola

tests/server/server-executeQuery-012.phpt

@@ -1,5 +1,7 @@
 --TEST--
 MongoDB\Driver\Server::executeQuery() pins transaction to server
+--XFAIL--
+_mongoc_cursor_fetch_stream segfault (CDRIVER-4290)
 --SKIPIF--
 <?php require __DIR__ . "/../utils/basic-skipif.inc"; ?>
 <?php skip_if_not_mongos(); ?>