Add a dummy plan.

Add a dummy plan to avoid "No plans" error in CI when a pull-request has only
files such as `README.md`. In this case, the plan "/modified" was not detected
by `modified-only: true`, and CI failed with the "No plans" error.

plans.fmf

@@ -10,3 +10,15 @@ execute:
     modified-only: true
     modified-ref: reference/main
     modified-url: https://src.fedoraproject.org/tests/ruby
+
+# A dummy plan to avoid "No plans" error in CI when a pull-request has only
+# files not detected by the `modified-only: true`.
+/dummy:
+  summary: A dummy plan
+  discover:
+    how: shell
+    tests:
+      - name: /dummy
+        test: echo "Dummy"
+  provision:
+    how: local