Check that hook_image_toolkits() is called and only available toolkits are returned.
function testGetAvailableToolkits() {
  $toolkits = image_get_available_toolkits();
  $this
    ->assertTrue(isset($toolkits['test']), 'The working toolkit was returned.');
  $this
    ->assertFalse(isset($toolkits['broken']), 'The toolkit marked unavailable was not returned');
  $this
    ->assertToolkitOperationsCalled(array());
}