Partially revert "Adapt test cases for Oniguruma 6.9.4"

This partially reverts commit c55d09c,
because `MB_ONIGURUMA_VERSION` is only available as of PHP 7.4.0, so
that change made no sense for PHP-7.3; we keep it for PHP-7.4, though.
We also stick with the modification to bug78633.phpt.

Author: cmb69

ext/mbstring/tests/mb_ereg_search_named_subpatterns_1.phpt renamed to ext/mbstring/tests/mb_ereg_search_named_subpatterns.phpt

@@ -4,7 +4,6 @@ Testing mb_ereg_search() named capture groups
 <?php
 if (!extension_loaded('mbstring')) die('skip mbstring not enabled');
 function_exists('mb_ereg_search') or die("skip mb_ereg_search() is not available in this build");
-version_compare(MB_ONIGURUMA_VERSION, '6.9.4', '<') or die("skip requires oniguruma < 6.9.4");
 ?>
 --FILE--
 <?php