Disable system addon updates for Firefox in testing mode

This commit fixes the following log line that currently shows up for
every type of tests that involve a browser:

`System addon update list error SyntaxError: XMLHttpRequest.open:
'http://%(server)s/dummy-system-addons.xml' is not a valid URL.`

If Firefox is in testing mode, the system addons update URL is
configured to a dummy URL so that it can't actually update (see for
example the same value in Marionette at
https://searchfox.org/mozilla-central/source/testing/marionette/client/marionette_driver/geckoinstance.py#109),
but this doesn't stop Firefox from trying to update, and when it does
it logs this line because the URL is obviously invalid.

Hence this patch which disables system addon updates altogether so
Firefox doesn't attempt to use the dummy URL anymore. The browser
updates are all managed by Puppeteer, and regular updates have already
been disabled too (see
6937a76f0a/packages/browsers/src/browser-data/firefox.ts (L302-L303)).

test/test.mjs

@@ -916,6 +916,8 @@ async function startBrowser({
     options.protocol = "webDriverBiDi";
 
     options.extraPrefsFirefox = {
+      // Disable system addon updates.
+      "extensions.systemAddon.update.enabled": false,
       // avoid to have a prompt when leaving a page with a form
       "dom.disable_beforeunload": true,
       // Disable dialog when saving a pdf