Reduce memory limit for gh19983.phpt

This test takes 140s on my machine with ASAN, and thus cause a bunch of timeouts
in nightly. The issue reproduces with 16M, reducing execution time to 20s.

Also mark the tests as slow.

Author: iluuu1994

Zend/tests/fibers/gh19983.phpt

@@ -1,9 +1,12 @@
 --TEST--
 GH-19983 (GC Assertion Failure with fibers, generators and destructors)
 --SKIPIF--
-<?php if (PHP_INT_SIZE < 8) die("skip 64-bit only - fiber stacks exhaust 32-bit address space"); ?>
+<?php
+if (PHP_INT_SIZE < 8) die("skip 64-bit only - fiber stacks exhaust 32-bit address space");
+if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
+?>
 --INI--
-memory_limit=128M
+memory_limit=16M
 --FILE--
 <?php
 class a