Add display & network related pytest cases

[myselfhimself]

Found the need for the following test cases when assisting a MacPorts maintainer for building and test gmic-py:

• [ ] test for enabled display - by parsing of the print command output header or some other more clever technique. This will not check that a window shows up, but this adds a showstopper for any OS builders who care about user interfaces.

• [ ] test for working Internet download (eg. https://placekitten.com/ or the sample command), which bulletprooves existence and use of wget, curl or libcurl by gmic-py/libgmic.