Skip imap_lsub_basic.phpt under asan

LeakSanitizer fails on Azure with:

> Tracer caught signal 11: addr=0x0 pc=0x7f86cde7c8b0 sp=0x7f86c46bcce0

ext/imap/tests/imap_lsub_basic.phpt

@@ -5,6 +5,7 @@ Olivier Doucet
 --SKIPIF--
 <?php
 require_once(__DIR__.'/skipif.inc');
+if (getenv("SKIP_ASAN")) die("skip leak sanitizer crashes");
 ?>
 --FILE--
 <?php