[unittest] Disambiguate references to errno (#77155)
In Xcode 16 SDKs there seem to be two `errno` visible for some files,
one in the `Darwin` module, and one in the `_errno` module (which seems
new for this Xcode version).
Disambiguate the references by prepending `Darwin`, which should be the
one that was being used before Xcode 16 SDKs.
