Skip tests for old versions instead of marking them XFAIL

According to commit 0eea9a6[1], these tests fail with old Oniguruma
versions; we are not sure which version of Oniguruma is required to let
them pass, but at least 6.9.3 is sufficient.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=0eea9a642941ab5d4c612f8092f186977afbb73e>

Author: cmb69

ext/mbstring/tests/mbregex_stack_limit.phpt

@@ -1,9 +1,13 @@
 --TEST--
 Test oniguruma stack limit
 --SKIPIF--
-<?php extension_loaded('mbstring') or die('skip mbstring not available'); ?>
---XFAIL--
-Travis CI has old oniguruma library
+<?php
+extension_loaded('mbstring') or die('skip mbstring not available');
+if (!function_exists('mb_ereg')) die('skip mb_ereg not available');
+if (version_compare(MB_ONIGURUMA_VERSION, '6.9.3') < 0) {
+    die('skip requires Oniguruma 6.9.3');
+}
+?>
 --FILE--
 <?php
 $s = str_repeat(' ', 30000);

ext/mbstring/tests/mbregex_stack_limit2.phpt

@@ -1,9 +1,14 @@
 --TEST--
 Test oniguruma stack limit
 --SKIPIF--
-<?php extension_loaded('mbstring') or die('skip mbstring not available'); ?>
---XFAIL--
-Travis CI has old oniguruma library
+<?php
+extension_loaded('mbstring') or die('skip mbstring not available');
+extension_loaded('iconv') or die('skip iconv not available');
+if (!function_exists('mb_ereg_replace')) die('skip mb_ereg_replace not available');
+if (version_compare(MB_ONIGURUMA_VERSION, '6.9.3') < 0) {
+    die('skip requires Oniguruma 6.9.3');
+}
+?>
 --FILE--
 <?php
 function mb_trim( $string, $chars = "", $chars_array = array() )