The contents of the "tests" subdirectory in the MediaWiki 1.26.0 tarball are not current. The tests/phpunit/phpunit.php file does not include phar support, for example, which was implemented in 2014:
https://phabricator.wikimedia.org/rMW33ea33f7b6ed196343a7934926a24a0055cb87b2
As a result, any use of tests/phpunit/phpunit.php produces this error message:
PHP Fatal error: Class 'SebastianBergmann\Version' not found in /usr/share/php/PHPUnit/Runner/Version.php on line 74 PHP Stack trace: PHP 1. {main}() tests/phpunit/phpunit.php:0 PHP 2. PHPUnit_Runner_Version::id() tests/phpunit/phpunit.php:108