Since #1901670: Start using PHPUnit for unit tests the output of simpletest_script_get_test_list is expected to be something like:
array(
'group' => array(
'testclass1',
'testclass2',
),
);However if you use --module or --class then this function currently returns
array(
'testclass1',
'testclass2',
);as it used too. This means that simpletest_script_execute_batch flattens the test_group array to an empty array and no tests are run.
So for example, without this patch the following command is a total fail...
$ php ./core/scripts/run-tests.sh --class "Drupal\book\Tests\BookTest"