Bug 1689672 - Ensure docshell/test/mochitest/test_bug660404.html works with SHIP-BFCache r=nika

Depends on D109212

Differential Revision: https://phabricator.services.mozilla.com/D109213

docshell/test/mochitest/mochitest.ini

@@ -93,7 +93,6 @@ skip-if = fission # Bug 1667389
 [test_bug640387_2.html]
 [test_bug653741.html]
 [test_bug660404.html]
-skip-if = fission # bug 1666449
 [test_bug662170.html]
 [test_bug668513.html]
 support-files = file_bug668513.html

docshell/test/mochitest/test_bug660404.html

@@ -64,9 +64,11 @@ bc.onmessage = (msgEvent) => {
     }
 }
 
-
-// Have to open a new window, since there's no bfcache in subframes
-window.open("file_bug660404-1.html", "", "noopener");
+// If Fission is disabled, the pref is no-op.
+SpecialPowers.pushPrefEnv({set: [["fission.bfcacheInParent", true]]}, () => {
+  // Have to open a new window, since there's no bfcache in subframes
+  window.open("file_bug660404-1.html", "", "noopener");
+});
 
 </script>
 </pre>