Re-enable test_zuul_trigger_project_change_merged

It looks like this now just works (with a minor main.yaml fix). Thus
reenable this test to ensure a merge-check pipeline keeps working.

Change-Id: Id788d4979aa70b45773cededa9ac2a025246b259
diff --git a/tests/fixtures/config/zuultrigger/project-change-merged/main.yaml b/tests/fixtures/config/zuultrigger/project-change-merged/main.yaml
index 9d01f54..208e274 100644
--- a/tests/fixtures/config/zuultrigger/project-change-merged/main.yaml
+++ b/tests/fixtures/config/zuultrigger/project-change-merged/main.yaml
@@ -4,3 +4,5 @@
       gerrit:
         config-projects:
           - common-config
+        untrusted-projects:
+          - org/project