[WDR] Fix WDRMaus and update tests

[dirkf]

Please follow the guide below

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Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

According to issue #30484, support for the WDR children's mini-site 'Die Sendung mit der Maus' was broken.

This PR restores that (and possibly other) functionality and fixes the extractor tests.

A pseudo-URL wdr:id is supported, where id is the 6+ digit suffix in mdb-nnnnnn.

The obsolete WDRMobileIE is marked as not _WORKING.

Resolves #30484.

Also resolves/replaces these open issues/PRs:

• closes [wdr] add support for wdrmaus.de content pages as playlists #13178
• closes Cannot download from WDR #17989
• closes Sendung mit der Maus #16448.