Testing the password dialog of a NetworkManager VPN plugin is as simple as:
echo -e 'DATA_KEY=foo\nDATA_VAL=bar\nDONE\nQUIT\n' | ./auth-dialog/nm-iodine-auth-dialog -n test -u $(uuid) -i
The above is for the iodine plugin when run from the built source tree. This allows one to test these dialogs although one didn't see them since ages since GNOME shell uses the external UI mode to query for the password.
This blog is flattr enabled.