The 'BadGroupTest' class isn't loaded there, so Drupal dies with the following message:
"Fatal error: Call to undefined method BadGroupTest::getTestInstances() in /var/www/drupal/sites/all/modules/simpletest/simpletest.module on line 177"
In 'drupal_unit_tests.php' line 50 there seems to be some 'old' code (or copied from simpletest itself?) to display a message when no tests are defined.
$this->addTestCase(new BadGroupTest($test_file, 'No new test cases'));
I'd reckon that this should be changed to a 'drupal_set_message()' or something...