Add tests of using portal-userauthcookie to continue through gateway

Add tests of using portal-userauthcookie to continue through gateway

This test sets up fake-gp-server.py to require a one-time password on the
portal *and* on the gateway, and to set 'portal-userauthcookie' after
successful login to the gateway.  For OpenConnect to successfully login
within prompting for a second password, the 'portal-userauthcookie' value
from the portal must be forwarded to the gateway login request.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>