Disable network connections to Contile for the tests

In PR #18356 the new tab page logic was disabled to prevent Firefox
from logging failed network connections to Contile, the Mozilla Tiles
service that is used for the new tab page [1]. However, recently this
log reappeared locally and on the bots:

```
console.warn: TopSitesFeed: Failed to fetch data from Contile server:
NetworkError when attempting to fetch resource.
```

It looks like Contile communication is also triggered from other places
in Firefox such as the URL bar [2], so this commit fixes the issue by
disabling network connections to Contile [3] altogether regardless of
their origin within Firefox. Note that we don't revert the change from
PR #18356 because as noted in [4] it can't hurt to keep that disabled
too to avoid overhead for a feature we don't use in the tests.

[1] https://github.com/mozilla-services/contile
[2] 196ef8360e/browser/components/urlbar/UrlbarProviderTopSites.sys.mjs (L38)
[3] 196ef8360e/browser/components/newtab/lib/TopSitesFeed.sys.mjs (L111)
[4] https://github.com/mozilla/pdf.js/pull/18356#issuecomment-2200354730

test/test.mjs

@@ -944,6 +944,8 @@ async function startBrowser({
       "accessibility.browsewithcaret": true,
       // Disable the newtabpage stuff.
       "browser.newtabpage.enabled": false,
+      // Disable network connections to Contile.
+      "browser.topsites.contile.enabled": false,
       ...extraPrefsFirefox,
     };
   }