Followups for initial CI test.

Add missing tasks section to tests playbook.

Use an absolute path to the example script since 'dir' is always
relative to /var/str.

Must define 'dir' explicitly -- there is no default handling.

tests/tests.yml

@@ -6,9 +6,10 @@
         - classic
       tests:
         - list-devices:
-            dir: /usr/share/doc/python3-blivet/examples
-            run: python3 ./list_devices.py
+            dir: .
+            run: python3 /usr/share/doc/python3-blivet/examples/list_devices.py
 
+  tasks:
     - name: copy out blivet logs
       fetch:
         dest: "{{ artifacts }}"