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/merge-failure.yaml b/tests/fixtures/layouts/merge-failure.yaml
index cc683a9..7c5121c 100644
--- a/tests/fixtures/layouts/merge-failure.yaml
+++ b/tests/fixtures/layouts/merge-failure.yaml
@@ -47,6 +47,10 @@
     precedence: high
 
 - job:
+    name: base
+    parent: null
+
+- job:
     name: project-merge
     hold-following-changes: true