Bug 1874875 - set clonable attribute check to only pass in Nightly. r=saschanaz DONTBUILD

The 'clonable' attribute is only available in Nightly (see bug 1868428).

Differential Revision: https://phabricator.services.mozilla.com/D198783

testing/web-platform/meta/dom/idlharness.window.js.ini

@@ -10,3 +10,7 @@
 
   [AbortSignal interface: calling any(sequence<AbortSignal>) on new AbortController().signal with too few arguments must throw TypeError]
     expected: FAIL
+
+  [ShadowRoot interface: attribute clonable]
+    expected:
+      if release_or_beta: FAIL