Make pipeline approval username requirement a regular expression

This now matches the trigger username requirement which is already
a regex.

Change-Id: Ib88998de9057f6605b8bbb6c52ab9bd2486b2e83
diff --git a/tests/fixtures/layout-requirement-username.yaml b/tests/fixtures/layout-requirement-username.yaml
index 7a549f0..f9e6477 100644
--- a/tests/fixtures/layout-requirement-username.yaml
+++ b/tests/fixtures/layout-requirement-username.yaml
@@ -3,7 +3,7 @@
     manager: IndependentPipelineManager
     require:
       approval:
-        - username: jenkins
+        - username: ^(jenkins|zuul)$
     trigger:
       gerrit:
         - event: comment-added