Make check that require a network connection optional

[mnhauke]

I am one of the openSUSE check_dmarc package maintainers.

We usually run all the unit tests during the build but since we don't have a network connection in the OBS environment during the build we use the following patch to skip the tests that require a network connection.

https://build.opensuse.org/projects/home:mnhauke/packages/python-checkdmarc/files/skip-network-tests.patch?expand=1