Hi, @Karun Khanna
Thank you for posting in Microsoft Q&A forum.
There will be no issue that extend schema in test domain.
ConfigMgr don't rely on AD in any way for client management, so multiple domains or forests with or without trusts are irrelevant.
Here is a great article by Jason about this:
https://www.1e.com/news-insights/blogs/mvp-questions-answered/
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