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-mutex.yaml b/tests/fixtures/layout-mutex.yaml
new file mode 100644
index 0000000..fcd0529
--- /dev/null
+++ b/tests/fixtures/layout-mutex.yaml
@@ -0,0 +1,25 @@
+pipelines:
+  - name: check
+    manager: IndependentPipelineManager
+    trigger:
+      gerrit:
+        - event: patchset-created
+    success:
+      gerrit:
+        verified: 1
+    failure:
+      gerrit:
+        verified: -1
+
+jobs:
+  - name: mutex-one
+    mutex: test-mutex
+  - name: mutex-two
+    mutex: test-mutex
+
+projects:
+  - name: org/project
+    check:
+      - project-test1
+      - mutex-one
+      - mutex-two