cap_net tests: Skip tests if there is no connectivity

When testing cap_connect() and name/addr lookup functions, skip tests if
we fail and the error is not ENOTCAPABLE. This makes the tests amenable
to running in CI without Internet connectivity.

Reviewed by: oshogbo
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D39242

(cherry picked from commit 7ee4066db1295fe3660964c2561bef87da7431f8)