[ie/HearThisAt] Improve _VALID_URL

[bohwaz]

Description of your pull request and other information

This updates the HearThis.at URL regexp to allow for private tracks, which contain a last part:

Private track URL: https://hearthis.at/tindalos/0001-tindalos-gnrique/eQd/
Public track URL: https://hearthis.at/moofi/dr-kreep

Closes #9755

Template

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[bashonly]

Can you add a test for a private track URL?