Update unit test regex

[bwalkerl]

There was 1 error:

1) tool_advancedreplace\helper_test::test_regex_search
Undefined index: page

/var/www/site/admin/tool/advancedreplace/tests/helper_test.php:370
/var/www/site/lib/phpunit/classes/advanced_testcase.php:80
phpvfscomposer:///var/www/site/vendor/phpunit/phpunit/phpunit:97

The previous regex was not compatible with older versions of mysql (like 5.7), which was causing this error in a few environments. There should be no harm in changing the regex to a simplified version.