Add incremental upgrades on the test channel for Tails 4.18

wiki/src/upgrade/v2/Tails/4.17/amd64/test/upgrades.yml

+---
+build-target: amd64
+channel: test
+initial-install-version: '4.17'
+product-name: Tails
+upgrades:
+- details-url: https://tails.boum.org/news/version_4.18/
+  type: major
+  upgrade-paths:
+  - target-files:
+    - sha256: 3f641eb6a735b4db48236b91455e1183b9841454c66e2b3aecfa1efb363c5255
+      size: 1176215552
+      url: http://dl.amnesia.boum.org/tails/stable/tails-amd64-4.18/tails-amd64-4.18.iso
+    type: full
+  - target-files:
+    - sha256: 8a96e6fec28f6aec82bec77272bd677e305112b806e335a30cb6369967b735b8
+      size: 405110784
+      url: http://dl.amnesia.boum.org/tails/stable/iuk/v2/Tails_amd64_4.17_to_4.18.iuk
+    type: incremental
+  version: '4.18'

wiki/src/upgrade/v2/Tails/4.17/amd64/test/upgrades.yml.pgp

+-----BEGIN PGP SIGNATURE-----
+
+iHUEABYKAB0WIQTNTUNRr6aTP1dKmvuQsrS9eu0jXwUCYH3WjQAKCRCQsrS9eu0j
+X9+uAP9aXq8hxA+a2n/I7hzPQyd1dcYc0Z1R+DmqknHnxV++rwD+JR0OS98toO4/
+rTaDNMdInk6d2bPmr4fgq6gowqUt9gc=
+=6IT2
+-----END PGP SIGNATURE-----