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/nonvoting-job.yaml b/tests/fixtures/layouts/nonvoting-job.yaml
index c536e11..6a912bf 100644
--- a/tests/fixtures/layouts/nonvoting-job.yaml
+++ b/tests/fixtures/layouts/nonvoting-job.yaml
@@ -20,6 +20,10 @@
     precedence: high
 
 - job:
+    name: base
+    parent: null
+
+- job:
     name: nonvoting-project-merge
     hold-following-changes: true