test: add empty /etc/resolv.conf in test image

Portable services have a BindPath pointing to it, so it needs to
be available in the image

test/test-functions

@@ -1106,6 +1106,7 @@ install_config_files() {
     # we want an empty environment
     > $initdir/etc/environment
     > $initdir/etc/machine-id
+    > $initdir/etc/resolv.conf
 
     # set the hostname
     echo systemd-testsuite > $initdir/etc/hostname