Reset URL service between test boots

refs https://github.com/TryGhost/Toolbox/issues/592

- we should reset the URL service to avoid event listeners piling up and
  slowing down CI due to the number of events it has to process

ghost/core/test/utils/e2e-framework.js

@@ -73,6 +73,9 @@ const startGhost = async (options = {}) => {
     // Adapter cache has to be cleared to avoid reusing cached adapter instances between restarts
     adapterManager.clearCache();
 
+    // Reset the URL service so we clear out all the listeners
+    urlServiceUtils.resetGenerators();
+
     const defaults = {
         backend: true,
         frontend: false,