Make img sizes parsing test match MQ spec, add more tests

http://lists.w3.org/Archives/Public/www-style/2014Oct/0117.html
http://krijnhoetmer.nl/irc-logs/css/20141008#l-293
"no top-level semicolons", and in this context, the immediate contents of the parens are "top-level", so this actually fails the grammar check and gets thrown out as an invalid candidate.  Should be `not (;) 100vw, 1px`.