Bug 1685869, enable sync XHR tests also in beta, r=sefeng

Differential Revision: https://phabricator.services.mozilla.com/D101343

dom/xhr/tests/browser.ini

@@ -9,6 +9,6 @@ support-files =
   empty.html
   empty_parent.html
   slow.sjs
-fail-if = !e10s || release_or_beta
+fail-if = !e10s
 [browser_temporaryFile.js]
 support-files =  temporaryFileBlob.sjs

dom/xhr/tests/mochitest.ini

@@ -111,7 +111,7 @@ skip-if = toolkit == "android" && debug && !is_fennec
 [test_sync_xhr_event_handling.html]
 support-files =
   file_sync_xhr_event_handling_helper.html
-fail-if = !e10s || release_or_beta
+fail-if = !e10s
 [test_sync_xhr_nested.html]
 support-files =
   file_sync_xhr_nested_helper.html