Add simple_layout test decorator

And add it to test_no_job_project.

So that we can reduce the number of boilerplate git repos initialized
for every (or most) tests, make it simple to specify a single simple
layout file (like we used to do in zuul v2).  The more complicated
scenarios can rely on the full copy-git-repos-from-test-fixtures
behavior, but simple tests can have that automatically generated from
a simple zuul.yaml file.

Change-Id: Ibeec4c526a1097823589f2c38a50e40dc346e0e5

tests/fixtures/config/single-tenant/git/common-config/zuul.yaml

@@ -225,9 +225,3 @@
             dependencies: nonvoting-project-merge
         - nonvoting-project-test2:
             dependencies: nonvoting-project-merge
-
-- project:
-    name: org/no-jobs-project
-    check:
-      jobs:
-        - project-testfile

tests/fixtures/config/single-tenant/git/org_no-jobs-project/README

@@ -1 +0,0 @@
-staypuft

tests/fixtures/config/single-tenant/main.yaml

@@ -20,4 +20,3 @@
           - org/conflict-project
           - org/noop-project
           - org/experimental-project
-          - org/no-jobs-project