Adjust groups to reduce test dependencies (#1748)

Switch around the groups defined in the Gemfile to allow execution
without installing additional tools that are useful for development but
not needed for testing or packaging.

spec/unit/util/network_util_spec.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'rexml/document'
+
 require_relative '../../spec_helper'
 
 require 'vagrant-libvirt/util/network_util'