Implement -O and -G tests

[theclapp]

Implement -O (syntax.TsUsrOwn, "file exists and is owned by the effective user id") and -G (syntax.TsGrpOwn, "file exists and is owned by the effective group id") tests, for non-Windows only.

Under Windows, still panics, as before.

-O and -G are hard/not applicable for Windows, since it doesn't really have the concept of "file owners", only ACLs, and it's not clear how to translate one to the other.