test(inspec): update `system.rb` from template with gentoo support

test/integration/share/libraries/system.rb

       inspec.platform[:release].gsub(/2018.*/, '1')
     when 'arch'
       'base-latest'
+    when 'gentoo'
+      "#{inspec.platform[:release].split('.')[0]}-#{derive_gentoo_init_system}"
     else
       inspec.platform[:release]
     end
   end
 
+  def derive_gentoo_init_system
+    case inspec.command('systemctl').exist?
+    when true
+      'sysd'
+    else
+      'sysv'
+    end
+  end
+
   def build_platform_finger
     "#{build_platform_name}-#{build_finger_release}"
   end