Fix ipfix buffer use and mirror test

[themiron]

Without raw capabilities test does nothing, but with them fails later on buffer put with short crafted body or IPv4 send permission in IPv6-only environment

[swills]

The buffer fix test makes sense in ipfix_unix.go makes sense, I was able to reproduce that issue and confirm that change fixes it, but I did have to also set opts.IPFIXUDPSize inside the test.

I'm unsure about the changes to the test in ipfix_test.go. When I pull those in, I get a timeout on the test. I suspect this is an environment issue. I have IPv6 addresses setup and tried removing the v4 addresses on both loop-back and other interfaces in my test environment, but didn't have any luck. Can you explain more about your IPv6-only test environment that triggers this issue?