libc/sys: add errno test

Despite looking trivial, it requires proper split of exports from libsys
and libc, proper filtering work in rtld, and operational libsys.

Reviewed by: emaste, imp
Sponsored

libc/sys: add errno test

Despite looking trivial, it requires proper split of exports from libsys
and libc, proper filtering work in rtld, and operational libsys.

Reviewed by: emaste, imp
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D44075

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