Prepare for removal of PHPUnit/DbUnit

DbUnit was deprecated recently https://github.com/sebastianbergmann/dbunit/issues/217

We need to prepare for its pending removal upon the release of PHPUnit 8.

The only uses are:

1. Directly:
   1. lib/phpunit/classes/arraydataset.php
   2. lib/tests/statslib_test.php
2. advanced_testcase::createFlatXMLDataSet
   (Unused in core)
3. advanced_testcase::createXMLDataSet
   1. lib/tests/statslib_test.php
4. advanced_testcase::createCsvDataSet
   1. mod/quiz/tests/attempt_walkthrough_from_csv_test.php
   2. mod/data/tests/search_test.php
5. advanced_testcase::loadDataSet
   1. mod/data/tests/search_test.php
6. advanced_testcase::createArrayDataSet
   (Unused in core)
7. References to DbUnit:
   1. mod/quiz/report/responses/tests/responses_from_steps_walkthrough_test.php
   2. mod/quiz/report/statistics/tests/stats_from_steps_walkthrough_test.php
   3. mod/quiz/tests/attempt_walkthrough_from_csv_test.php

Pinging Tim Hunt for your info on this .