Bug 1636476: Enable a11y logging for accessible/tests/browser/events. r=morgan

I can't reproduce this on try, so I'm landing this so I can get (hopefully) useful info next time it fails.

Differential Revision: https://phabricator.services.mozilla.com/D115455

accessible/tests/browser/events/browser.ini

@@ -4,6 +4,8 @@ support-files =
   !/accessible/tests/browser/shared-head.js
   !/accessible/tests/mochitest/*.js
   !/accessible/tests/browser/*.jsm
+environment = 
+  A11YLOG=doclifecycle,events,notifications
 
 [browser_test_docload.js]
 skip-if = e10s