Create a file and munge/unmunge the name.
function testMunging() {
// Disable insecure uploads.
variable_set('allow_insecure_uploads', 0);
$munged_name = file_munge_filename($this->name, '', TRUE);
$messages = drupal_get_messages();
$this
->assertTrue(in_array(t('For security reasons, your upload has been renamed to %filename.', array(
'%filename' => $munged_name,
)), $messages['status']), 'Alert properly set when a file is renamed.');
$this
->assertNotEqual($munged_name, $this->name, format_string('The new filename (%munged) has been modified from the original (%original)', array(
'%munged' => $munged_name,
'%original' => $this->name,
)));
}