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

tests/fixtures/zuul-executor-hostname.conf

@@ -16,7 +16,7 @@ git_user_name=zuul
 
 [executor]
 git_dir=/tmp/zuul-test/executor-git
-hostname=test-executor-hostname.openstack.org
+hostname=test-executor-hostname.example.com
 
 [connection gerrit]
 driver=gerrit

tests/unit/test_executor.py

@@ -408,5 +408,5 @@ class TestExecutorHostname(ZuulTestCase):
     tenant_config_file = 'config/single-tenant/main.yaml'
 
     def test_executor_hostname(self):
-        self.assertEqual('test-executor-hostname.openstack.org',
+        self.assertEqual('test-executor-hostname.example.com',
                          self.executor_server.hostname)