Require a base job

This makes base jobs required and allows for a per-tenant default.

Story: 2001110
Task: 4793
Change-Id: I26ffddad8358c156cfac749ce98af70f3447f671
diff --git a/tests/fixtures/layouts/timer-smtp.yaml b/tests/fixtures/layouts/timer-smtp.yaml
index 66e9aaf..a27b183 100644
--- a/tests/fixtures/layouts/timer-smtp.yaml
+++ b/tests/fixtures/layouts/timer-smtp.yaml
@@ -11,6 +11,10 @@
         subject: Periodic check for {change.project} succeeded
 
 - job:
+    name: base
+    parent: null
+
+- job:
     name: project-bitrot-stable-old
     success-url: http://logs.example.com/{job.name}/{build.number}