Change domain in test fixture to example.com

This should be a domain defined in RFC 6761 which is specifically
reserved for documentation usage.

Change-Id: Ie681475d10f8d7cc910593e4ee41a1350acd3ef1
diff --git a/tests/fixtures/zuul-executor-hostname.conf b/tests/fixtures/zuul-executor-hostname.conf
index 7db144d..8199aba 100644
--- a/tests/fixtures/zuul-executor-hostname.conf
+++ b/tests/fixtures/zuul-executor-hostname.conf
@@ -16,7 +16,7 @@
 
 [executor]
 git_dir=/tmp/zuul-test/executor-git
-hostname=test-executor-hostname.openstack.org
+hostname=test-executor-hostname.example.com
 
 [connection gerrit]
 driver=gerrit