Heh, this is a question addressed in fiction some.
There’s a qri, Laurell K Hamilton, that has a character that’s a necromancer that raises zombies for legal issues as part of her job.
And, while it may seem derivative (and I admit it is to a degree), My urban fantasy series is based around a forensic necromancer, who raises spirits, zombies, and otherwise applies his magic to legal issues.
Back in the day, I spent a couple years talking to cops, lawyers, and a few medical examiners in preparation for doing writing. I used that to address their objections to the idea in my stuff.
But, yeah, if certain barriers could be overcome, courts could accept testimony from an undead entity. It would likely end up appealed and chained challenged up to the top court in any given country, bit it’s possible