Review incorrect phpunit_util uses and replace them by correct advanced_testcase extra methods

While reviewing MDL-46239 it was detected that various "low level" phpunit_util::xxxxx() methods were being used in testcases.

Looking globally lots of those uses were found, for example:

$ ag 'reset_debugging|get_debugging_messages' | wc -l

72

This issue is about to replace all them by the higher level extra function available @ advanced_testcase. Reference:

https://docs.moodle.org/dev/PHPUnit_integration#Extra_methods

Ciao