Refactor OsInspector

* Use consistent names for all methods which get information about
  the operating system
* Make logic of os inspection more explicit
* Use methods instead of hash elements to access attributes of Os
* Group tests according to which function they test
It's not an accessor of a variable, where Ruby indeed doesn't use the prefix. But it is an action where information is gotten. To make this clear the prefix is used there consistently now. The idea is that `get_` means getting something from the system, not just from an internal variable.