[MM-27967] api4/config_test: fix flaky test if siteurl is already set (#15403)

* api4/config_test: fix flaky test if siteurl is already set

* api4/apitelib enforce siteurl not set

api4/apitestlib.go

@@ -117,6 +117,7 @@ func setupTestHelper(dbStore store.Store, searchEngine *searchengine.Broker, ent
 		*cfg.TeamSettings.MaxUsersPerTeam = 50
 		*cfg.RateLimitSettings.Enable = false
 		*cfg.EmailSettings.SendEmailNotifications = true
+		*cfg.ServiceSettings.SiteURL = ""
 
 		// Disable sniffing, otherwise elastic client fails to connect to docker node
 		// More details: https://github.com/olivere/elastic/wiki/Sniffing