[TASK] Streamline AssetCollector ViewHelper tests

Just like other ViewHelper tests, the AssetCollector ViewHelper tests
now utilize the RenderingContext and TemplateView to render a real Fluid
snippet. The AssetCollector Singleton can now also be retrieved from
the DI container.

Resolves: #100290
Releases: main
Change-Id: I22841b46061d9f1510bc2967545fe8773b1b2ac3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78245


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Benni Mack <benni@typo3.org>