Set window location for individual test

This is less surprising and means we don't end up in a situation
where a future test needs to change the global config to something
else.

tests/javascripts/jest.config.js

@@ -1,4 +1,4 @@
 module.exports = {
   setupFiles: ['./support/setup.js'],
-  testURL: 'https://www.notifications.service.gov.uk/?next=%2Ffoo%3Fbar%3Dbaz'
+  testURL: 'https://www.notifications.service.gov.uk',
 }