github: unit-tests: install graphviz

This allows testing of the GraphStrategy graph rendering example in the
development documentation. If graphviz is not installed, the
corresponding test is skipped.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

.github/workflows/unit-tests.yml

@@ -25,7 +25,7 @@ jobs:
           ${{ runner.os }}-pip-
     - name: Install system dependencies
       run: |
-        sudo apt-get install -yq libow-dev openssh-server openssh-client libsnappy-dev ncurses-term
+        sudo apt-get install -yq libow-dev openssh-server openssh-client libsnappy-dev ncurses-term graphviz
         sudo mkdir -p /var/cache/labgrid/runner && sudo chown runner /var/cache/labgrid/runner
     - name: Prepare local SSH
       run: |