Add explicit tests for keywords incrementalism in domains

[ferringb]

Quoting portage manpage:

                     Example:
                     # always use unstable libgd
                     media-libs/libgd ~x86
                     # only use stable mplayer
                     media-video/mplayer -~x86
                     # always use unstable netcat
                     net-analyzer/netcat

In particular, note the -~x86. That's incremental logic, chained negation. I'm not having an easy time confirming domain is doing this correctly, but in tracing the code, it doesn't look like the pathways involved enable incremental logic.

Either way, add tests to extend confidence for this functionality, and/or flush out if it needs fixing.