[ci] Add graphviz to dev.yml for beat.cmdeline tests

graphviz is used by beat.core to generate dot diagrams. It's not
listed as dependency of that package as this feature has a narrow
use that would pull unnecessary dependencies for the most common
cases.

dev.yml

@@ -14,6 +14,7 @@ dependencies:
   - docopt
   - docutils
   - jinja2
+  - graphviz
   - matplotlib
   - psutil
   - psycopg2