Add job mutex support

This is so that jobs that interact with external resources can be
mutexed.

Change-Id: I94365e258cae30c5fe61981eccc879f400b02f7f
diff --git a/tests/fixtures/layout.yaml b/tests/fixtures/layout.yaml
index 1d23443..e8f035e 100644
--- a/tests/fixtures/layout.yaml
+++ b/tests/fixtures/layout.yaml
@@ -116,6 +116,10 @@
     parameter-function: select_debian_node
   - name: project1-project2-integration
     queue-name: integration
+  - name: mutex-one
+    mutex: test-mutex
+  - name: mutex-two
+    mutex: test-mutex
 
 project-templates:
   - name: test-one-and-two