Don't ignore inexistent jobs in config

Currently, when a project pipeline invokes a job which otherwise
doesn't exist, zuul simply runs nothing. As this is not the desired
behavior add a check during configuration time.

This also uncovers some bugs in the tests which are also fixed.

Change-Id: I37da435fdf3281f98ca097d79d504c170c13bf3a
Story: 2000893
diff --git a/tests/fixtures/layouts/reporting-multiple-github.yaml b/tests/fixtures/layouts/reporting-multiple-github.yaml
index f14000e..22fa1e7 100644
--- a/tests/fixtures/layouts/reporting-multiple-github.yaml
+++ b/tests/fixtures/layouts/reporting-multiple-github.yaml
@@ -25,7 +25,7 @@
 - job:
     name: project1-test1
 - job:
-    name: project2-test1
+    name: project2-test2
 
 - project:
     name: org/project1