KTEST: Support 'unixOnly' attributes in config.xml

I implemented the 'unixOnly' feature as Dwight asked as follows. It would be great if Omer also reviews this. 

On Feb 27, 2014, at 2:20 PM, Dwight Guth <dwight.guth@runtimeverification.com> wrote:

It recently came to my attention that some of the ktest tests for the Java rewrite engine got commented out. One in particular I am unable to add back in because a feature is missing from ktest. I'm not sure who deleted it, but it needs to be added back in.

Take a look at the tests/regression/java-rewrite-engine/io/test1 folder. It contains a file prepare.sh. Because of limitations of git, it's necessary that this script run on unix before these tests, in order for it to succeed. And because of limitations on IO portability between linux and windows, it's necessary that these tests only run on unix-based OSes.

Can you please add back in the functionality which allows me to specify this information in the config xml file?