[BUGFIX] Speed up linkvalidator functional tests

The ext:linkvalidator functional tests work with a
fictional domain name to test if the code finds broken
external links.

This can be unfortunate since it triggers DNS lookups
that can be slow - such stuff should be avoided in
functional tests anyway.

Solution is to use localhost as test domain with an
invalid path instead. This is more expressive, too.

The result can be stunning: My DNS resolver for whatever
reason takes ages to (not) resolve that domain using
dockerized runTests.sh. I got a similar result from
another coredev. The linkvalidator tests run nearly
five minutes without the patch and are down to two
seconds with patch.

Resolves: #96087
Releases: master, 11.5
Change-Id: Idbb32ac57925c52d4cef9e8d412d56347f09a04e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72312


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