[BUGFIX] Fix the asset paths in single HTMl

And add an integration test for single HTML.

It was until now not possible testing it.

There are still open issues with single HTMl, but I would suggest to solve them in follow-ups.

- TYPO3-Documentation/render-guides#238
- TYPO3-Documentation/render-guides#239

Resolves TYPO3-Documentation/render-guides#233

src/Renderer/SinglePageRenderer.php

@@ -22,7 +22,8 @@ public function render(RenderCommand $renderCommand): void
             $renderCommand->getDestination(),
             $renderCommand->getDestinationPath(),
             'singlepage',
-        )->withIterator($renderCommand->getDocumentIterator());
+        )->withIterator($renderCommand->getDocumentIterator())
+        ->withOutputFilePath('singlehtml/Index.html');
 
         $context->getDestination()->put(
             $renderCommand->getDestinationPath() . '/singlehtml/Index.html',