Set jest testURL to https://design-system.service.gov.uk

Without this config, the default URL is 'localhost' which causes
some issues when setting cookies because of a bug in tough-cookie which treats
localhost like a public domain. Because you can't set cookies on a public domain,
cookies were not being set in the tests.

Setting the testURL to a real URL fixes this issue.

@lfdebrux has raised an issue with tough-cookie here: salesforce/tough-cookie#215

package.json

@@ -88,7 +88,8 @@
     "preset": "jest-puppeteer",
     "setupFilesAfterEnv": [
       "./jest.setup.js"
-    ]
+    ],
+    "testURL": "https://design-system.service.gov.uk"
   },
   "browserslist": [
     "last 2 versions",