How can I mock a method which is not defined in the interface?

In my php file I am using Zend\Cache\Storage\StorageInterface. I have bind this interface with Zend\filesystem cache. I have to unit test the main php file. Problem is StorageInterface does not define flush method. Filesystem have flush method from different interface.

In my unit testing I am trying to mock the StorageInterface but problem is it gives an error stating that flush is not defined. How can I stub the flush method?

Binding:

$container->bindIf(
            "Zend\\Cache\\Storage\\StorageInterface",
            function () {
                $directoryPath = "/tmp/zend_report_cache";
                if (!file_exists($directoryPath) && !mkdir($directoryPath)) {
                    throw new Exception("Failed to create directory [directoryPath=$directoryPath]");
                }
                return \Zend\Cache\StorageFactory::factory(array(
                    'adapter' => array(
                        'name' => 'filesystem',
                        'options' => array(
                            'cache_dir' => $directoryPath
                        ),
                    ),
                    'plugins' => array('serializer'),
                ));
            }
        );

Main.php (only showing relevant code

use Zend\Cache\Storage\StorageInterface;

public function __destruct()
{
    $this->cache->flush();
}

unit test file

private function getCacheMock()
    {
$cacheMock = $this->setMethods(['execute'])->createMock(StorageInterface::class);
        $cacheMock->expects($this->at(0))->method('setItem')->with(CsvReport::HEADER_CACHE_KEY, []);
        $cacheMock->expects($this->at(1))->method('getItem')->with(CsvReport::HEADER_CACHE_KEY)->willReturn([]);
        $cacheMock->expects($this->at(2))->method('setItem')->with(CsvReport::HEADER_CACHE_KEY, ['key1', 'key2', 'key3']);
        $cacheMock->expects($this->at(3))->method('setItem')->with(0, $this->getMockRow(0));
        $cacheMock->expects($this->at(4))->method('getItem')->with(CsvReport::HEADER_CACHE_KEY)->willReturn(['key1', 'key2', 'key3']);
        $cacheMock->expects($this->at(5))->method('setItem')->with(CsvReport::HEADER_CACHE_KEY, ['key1', 'key2', 'key3', 'key4']);
        $cacheMock->expects($this->at(6))->method('setItem')->with(1, $this->getMockRow(1));
        $cacheMock->expects($this->at(7))->method('getItem')->with(CsvReport::HEADER_CACHE_KEY)->willReturn(['key1', 'key2', 'key3', 'key4']);
        $cacheMock->expects($this->at(8))->method('setItem')->with(CsvReport::HEADER_CACHE_KEY, ['key1', 'key2', 'key3', 'key4', 'key5']);
        $cacheMock->expects($this->at(9))->method('setItem')->with(2, $this->getMockRow(2));
        return $cacheMock;
    }

Error: Trying to configure method "flush" which cannot be configured because it does not exist, has not been specified, is final, or is static