libc/limits_test: add no-op testcase to satisfy kyua

This test suite is purely tested with compile-time assertions, so
it needs a dummy runtime test to ensure that kyua reports the
file as passing.

Pull Request: https://github.com/freebsd/freebsd-src/pull/1915
Sponsored by: The FreeBSD Foundation
Reviewed by: fuz
Approved by: markj (mentor)
MFC after: 1 month
Signed-off-by: Siva Mahadevan <me@svmhdvn.name>