[RFR] Fix cache image generation on parsedown assert

[jnadaud]

When image cache is enable and you are trying to assert image from parsedown, the path is not the same as original but it's a generated one from cache

$this->assertSame('<p><img alt="" src="/subdir/tests/fake/nestedsite/user/pages/02.item2/02.item2-2/sample-image.jpg" /></p>',
            $this->parsedown->text('![](sample-image.jpg)'))