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());
}