I guess the easiest way (off the top of my head) might be to have a single test that re-cursed through your app/ directory (or whatever other ones you wanted) and did a file_get_contents() for each .php file and then assert it doesn't contain 'bad' debug left-overs like var_dump/dd( etc. Maybe something like :
$appDir = new RecursiveDirectoryIterator('app/');
$iterator = new RecursiveIteratorIterator($appDir);
$matches = new RegexIterator($iterator, '/^.+\.php$/i', RecursiveRegexIterator::GET_MATCH);
foreach ($matches as $phpFile) {
$contents = file_get_contents($phpFile);
$this->assertFalse(preg_match('/(var_dump|dd\()/s', $contents));
}
Something like that anyway :-)