Bug 1636783 - Ensure default profile permissions do not confuse permission manager tests (test_permmgr.js). r=mayhemer,necko-reviewers

This is a problem for Thunderbird due to bug 1168178.

Differential Revision: https://phabricator.services.mozilla.com/D75031

netwerk/test/unit/head_cookies.js

@@ -7,11 +7,13 @@
 "use strict";
 
 const { NetUtil } = ChromeUtils.import("resource://gre/modules/NetUtil.jsm");
-
 const { CookieXPCShellUtils } = ChromeUtils.import(
   "resource://testing-common/CookieXPCShellUtils.jsm"
 );
 
+// Don't pick up default permissions from profile.
+Services.prefs.setCharPref("permissions.manager.defaultsUrl", "");
+
 CookieXPCShellUtils.init(this);
 
 XPCOMUtils.defineLazyServiceGetter(