Bug 1582053 - Disable the remote settings server in web extension xpcshell tests; r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D46498

--HG--
extra : moz-landing-system : lando

toolkit/components/extensions/test/xpcshell/head.js

@@ -46,6 +46,11 @@ const testEnv = {
   expectRemote: false,
 };
 
+Services.prefs.setStringPref(
+  "services.settings.server",
+  "http://localhost:7777/remote-settings-dummy/v1"
+);
+
 add_task(function check_remote() {
   Assert.equal(
     WebExtensionPolicy.useRemoteWebExtensions,