Add playbook support to simple_layout in tests

All jobs need playbooks, so make sure that we create a stub playbook
for every job which appears in a simple layout.

Move the test_nonvoting_pipeline test to use simple_layout.

Change-Id: Id207fa3c1d01bfaef49f51c86589781b7892585c

tests/fixtures/layouts/nonvoting-pipeline.yaml

@@ -0,0 +1,25 @@
+- pipeline:
+    name: experimental
+    manager: independent
+    trigger:
+      gerrit:
+        - event: patchset-created
+    success:
+      gerrit: {}
+    failure:
+      gerrit: {}
+
+- job:
+    name: project-merge
+    hold-following-changes: true
+
+- job:
+    name: experimental-project-test
+
+- project:
+    name: org/experimental-project
+    experimental:
+      jobs:
+        - project-merge
+        - experimental-project-test:
+            dependencies: project-merge