Non-resolvable Domain Names should be excluded from tests

[chriswhitingBCH]

We use internal only domains to route emails between exchange and our email signature management system. DMARC, SPF, etc. etc. should not be run on non-resolvable domains

[soulemike]

This one is a bit similar to #549 with the second being you are accepting risk that you trust that domain not to be spoofed by anyone. This is a little more likely to be manipulated too since .cloud is a TLD managed by ICANN, so it isn't an internal only specification.

Similarly though, open to suggestions on how to adequately warn of the risk while ignoring it.

[merill]

We should do the same as what we plan for #549