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/three-projects.yaml b/tests/fixtures/layouts/three-projects.yaml
index 6481fb9..51cd406 100644
--- a/tests/fixtures/layouts/three-projects.yaml
+++ b/tests/fixtures/layouts/three-projects.yaml
@@ -33,6 +33,10 @@
     precedence: high
 
 - job:
+    name: base
+    parent: null
+
+- job:
     name: project-merge
     hold-following-changes: true