Add incremental upgrades on the test channel for Tails 3.9~rc1

wiki/src/upgrade/v1/Tails/3.8/amd64/test/upgrades.yml

+---
+build-target: amd64
+channel: test
+product-name: Tails
+product-version: '3.8'
+upgrades:
+- details-url: https://tails.boum.org/news/test_3.9-rc1/
+  type: major
+  upgrade-paths:
+  - target-files:
+    - sha256: 743641f7093e2e597ebef39d14df4678db381aebd2ffe6d518c679fa5d921eb0
+      size: 1225568256
+      url: http://dl.amnesia.boum.org/tails/alpha/tails-amd64-3.9~rc1/tails-amd64-3.9~rc1.iso
+    type: full
+  - target-files:
+    - sha256: 2c5baba04429189209c610c5ac32962eb587c00a5da7dae91d379e52299af0b9
+      size: 376145920
+      url: http://dl.amnesia.boum.org/tails/alpha/iuk/Tails_amd64_3.8_to_3.9~rc1.iuk
+    type: incremental
+  version: 3.9~rc1

wiki/src/upgrade/v1/Tails/3.8/amd64/test/upgrades.yml.pgp

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