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
diff --git a/doc/source/developer/testing.rst b/doc/source/developer/testing.rst
index 4a813d0..057ab7e 100644
--- a/doc/source/developer/testing.rst
+++ b/doc/source/developer/testing.rst
@@ -9,6 +9,8 @@
 access to a number of attributes useful for manipulating or inspecting
 the environment being simulated in the test:
 
+.. autofunction:: tests.base.simple_layout
+
 .. autoclass:: tests.base.ZuulTestCase
    :members: