Bug 1282024 - Fix the typos that break setting the pref for iframe-allowfullscreen.html, and reenable it

testing/web-platform/meta/html/semantics/embedded-content/the-iframe-element/iframe-allowfullscreen.html.ini

@@ -1,4 +1,3 @@
-[iframe-allow-fullscreen.html.ini]
+[iframe-allowfullscreen.html]
   type: testharness
-  disabled: https://bugzilla.mozilla.org/show_bug.cgi?id=1282024
   prefs: [full-screen-api.unprefix.enabled:true]