Tweaked code coverage config for e2e tests

refs https://github.com/TryGhost/Toolbox/issues/475
refs https://github.com/TryGhost/Toolbox/issues/117

- The frontend/src folded would never get picked up by a code coverage tooling as scripts there are dynamically minimized and served from the server
- There's nothing to cover under frontend/public
- Logic behind these changes is the same as in second referenced issue

ghost/core/.c8rc.e2e.json

@@ -10,11 +10,13 @@
   "statements": 78,
   "branches": 80,
   "functions": 80,
-  "lines": 78,
+  "lines": 79,
   "include": [
     "core/{*.js,frontend,server,shared}"
   ],
   "exclude": [
+    "core/frontend/src/**",
+    "core/frontend/public/**",
     "core/server/data/migrations/**",
     "!core/server/data/migrations/utils.js"
   ]