IMO this should be treated as a new feature for the next release. But consistently returning the path sounds good to me if there is no good reason not to.

Left a question on the review. I think you also need to update the documentation, and since this is changing documented behaviour it probably needs a What’s New entry.

For tests, I would try doing something like find_library("c"), and ensuring that the result is an absolute path. We may have to end up skipping the test for platforms like Windows where this is not expected to work. Look through the files in /Lib/ctypes/test/ for a good place for it to live (if there isn’t already a test there you can modify).