cap_dns tests: Convert to ATF, avoid failing when lookups don't resolve

The cap_dns tests require Internet access. Currently they fail when
that's not available, which for CI purposes is undesirable. Let's
instead skip the tests if none of the non-casper name/addr lookups
succeed.

To that end:

• Convert the tests to ATF so that skipping is easier to implement.
• Break up the tests into separate test cases.
• If one of the system (i.e., non-casper) lookup functions fails, skip the test if all of them failed, otherwise fail the tests, since partial failure indicates something is flaky and deserves a closer look.

Reviewed by: oshogbo
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39241