added has_stat test

Hello,

i added a has_stat test. as its first argument it takes a filename, the second argument is a hashref with the fields
`owner` and `group`. if set it resolves this fields to `uid` and `gid` and checks if they match with the actual file owner/group.

one could add a bunch of other fields as well(uid, gid, size, atime, mtime, mode). for now i only needed these two, so i only implemented them.
I'm not entirely sure that returning OK is the best way to handle the case of missing files. Please convince me, or set it to fail :) Probably worth adding a `sub fail...` to Rex::Test::Base and then using that.