Bug 1627972: Re-enable test_upgrade_insecure.html on all platform. r=baku

Differential Revision: https://phabricator.services.mozilla.com/D80912

dom/security/test/csp/mochitest.ini

@@ -300,8 +300,6 @@ fail-if = xorigin
 [test_dual_header.html]
 [test_win_open_blocked.html]
 [test_upgrade_insecure.html]
-# no ssl support as well as websocket tests do not work (see test_websocket.html)
-skip-if = (os != 'linux' && !debug) # Bug 1183300
 [test_upgrade_insecure_reporting.html]
 [test_upgrade_insecure_cors.html]
 [test_upgrade_insecure_loopback.html]