Is there anywhere that I can get a valid , signed, SSL cert to the "localhost" domain for testing?
I have a really hard time believing that exists, since it's against the nature of SSL.
The CA's that are in your root-certificate list are companies who we've given our trust to. Their job is to verify that the ones who own the certificate are the ones they claim to be. Giving a localhost-certificate to anyone like this, would break this concept. Even for testing.
The localtest.me cert seems to be no longer available, but see https://github.com/Daplie/localhost.daplie.com-certificates. It's not a wildcard cert though.