Disable AI/ML features using Firefox preferences for testing

We don't need AI/ML features in the tests, so this should reduce CPU
usage by not having the inference process running. Moreover, it prevents
the following lines from being logged in the test output:

```
JavaScript error: resource://gre/actors/MLEngineParent.sys.mjs, line 509: Error: Unable to get the ML engine from Remote Settings.
JavaScript error: resource://gre/actors/MLEngineParent.sys.mjs, line 1279: TypeError: can't access property "postMessage", this[#port] is null
```

Author: timvandermeij

test/test.mjs

@@ -955,6 +955,12 @@ async function startBrowser({
       "browser.topsites.contile.enabled": false,
       // Disable logging for remote settings.
       "services.settings.loglevel": "off",
+      // Disable AI/ML functionality.
+      "browser.ml.enable": false,
+      "browser.ml.chat.enabled": false,
+      "browser.ml.linkPreview.enabled": false,
+      "browser.tabs.groups.smart.enabled": false,
+      "browser.tabs.groups.smart.userEnabled": false,
       ...extraPrefsFirefox,
     };
   }