Added some comments to evil_random_io in the unit tests, and tried to use some regex to improve our ability to test muliple compiler versions

I previously used GCCVER for the path stored in TESTPREFIX, but I'm trying to use some regular expressions to improve how we access the paths. We might be able to use this in a more robust way, and only have one PREFIXPATH by saying b.v2/libs/afio/test/*/boost_afio_test, or equivalent. Making this change might make the travis.yml less sensitve to changes.