Mark failing JIT test with XFAIL

Author: iluuu1994

ext/opcache/tests/jit/assign_dim_op_007.phpt

@@ -5,6 +5,8 @@ opcache.enable=1
 opcache.enable_cli=1
 opcache.file_update_protection=0
 opcache.jit_buffer_size=1M
+--XFAIL--
+See https://github.com/php/php-src/issues/8147
 --FILE--
 <?php
 class test {