Adding a pytest.mark.network marker

[rouault]

I see in https://github.com/rasterio/rasterio/pull/3193/files that rasterio uses such a marker to tag tests that depend on an internet connection. This could be a useful marker typically for packagers wanting to run GDAL regression test suite in environments without Internet access, or who want robust tests as Internet-based tests tend to be flaky We do have a pytest.mark.require_curl marker, but it is not quite the same, as we have tests that only require local network access as they stub a HTTP server Identifying all such tests might not be immediately trivial though. That might require to grep for "http://" / "https://" and analyzing case by case.