De-realstd os::args

De-realstd os::args

With the test runner using ::std::os::args(), and std::std::os now being
a re-export of realstd::os, there's no more need for realstd stuff
mucking up rt::args.

Remove the one test of os::args(), as it's not very useful and it won't
work anymore now that rt::args doesn't use realstd.