Add windows tests

[philpep]

Since we have some support for windows, we should have tests to ensure not breaking things. Maybe https://www.appveyor.com/ could provide this.

Since I'm not a windows expert, any help will be really appreciated.

[trevor-viljoen]

I'll look into this and see if I can figure something out.

[FilBot3]

Is there a TODO list for the features that have been added and are being worked on?

[aheumaier]

@philpep We are using testinfra in a customer setup where we use mixed docker + windows containers.
We would happily provide a windowscontainer backend tackling run command and file issues.

[dariuszparys]

@philpep I would take this one, feel free to assign me

[dariuszparys]

I had now a deeper look into the test configuration. To add windows support with the current code base would mean to have either a lot of specific branch code ior split into seperate folders/files inside the project and also split tox into platform specific pieces.

Even doing this would mean that there would be a lowered code coverage for the paths that are not available / runnable on Windows and Linux.

I'm unsure if either approach is one I would do