Inside of an extension create a parser tests file, name it parserTests.txt, and use $wgParserTestFiles[] = DIR . '/parserTests.txt';
Running parserTests will work perfectly fine. However attempting to run unit tests will result in the following.
PHP Fatal error: Cannot redeclare class ParserTests in {core}/tests/phpunit/includes/parser/MediaWikiParserTest.php(27) : eval()'d code on line 3
It appears that the phpunit parser test code makes really generic class names for parser test files based on the filename alone completely ignoring different paths.
Version: unspecified
Severity: minor