[rtlnl] update regex for new urlscheme

[ddland]

Before submitting a pull request make sure you have:

• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• Searched the bugtracker for similar pull requests
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

---

Description of your pull request and other information

The rtl.nl extractor does not work with the updated url-scheme anymore. This pull requests fixes the url-pattern. Without touching the old regex. A new testcase is added, old ones are still the same.
See also #25821

(tried a pull-request before, but #25816 got lost due to a rebase while updating to the most recent code-base).