test: fix missing IPv6 forwarding on tunnel in tunnel_basic

Enable IPv6 forwarding on the left DUT's tunnel interface, matching
the IPv4 tunnel config.  Without it, IPv6 packets are not forwarded
through the tunnel, causing all tunnel_basic IPv6 tests to fail.

Regression introduced when fixing issue #515 in 717c1c0.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/case/interfaces/tunnel_basic/test.py

@@ -115,7 +115,8 @@ with infamy.Test() as test:
                         "address": [{
                             "ip": "2001:db8:3c4d:30::1",
                             "prefix-length": 64
-                        }]
+                        }],
+                        "forwarding": True
                     },
                     CONTAINER_TYPE: container_left6
                 }]