MDL-71036 phpunit: Remove custom autoloader
Custom autoloaders are deprecated with PHPUnit 9 and will be removed with PHPUnit 10. Since PHPUnit 8.5 custom autoloaders don't do much because that version removed the ability to launch unit tests by class name and that's exactly the reason we had a custom autoloader (to map class names to files within our tests). See MDL-67673 about when direct use of classes was deprecated (8.5), now removed (9.5). So, as far as it's unused, removing it now, test still can be selectively using any of: - a relative path to file (although there are some restrictions comming with PHPUnit 9, see https://github.com/sebastianbergmann/phpunit/issues/4105 - using --filter, to point to any classname[::method] - using --testsuite to run a complete suite - using --config to point to custom components. Also, commented out the lib/ajax/tests directory because it doesn't exist / is empty and PHPUnit 9 emits error when a configured test directory does not exist. See https://github.com/sebastianbergmann/phpunit/issues/4493. Alternative was to completely remove the configuration line, but decided to keep it around in case some day we want to add some test there.