Alter the list of tests.
$groups: A two dimension array, the first key is the test group (as defined in getInfo) the second is the name of the class and the value is the return value of the getInfo method.
function hook_simpletest_alter(&$groups) {
// An alternative session handler module would not want to run the original
// Session HTTPS handling test because it checks the sessions table in the
// database.
unset($groups['Session']['testHttpsSession']);
}