Bug 1699941 - Ensure docshell/test/navigation/test_bug1379762.html works with SHIP-BFCache r=nika

Depends on D109216

Differential Revision: https://phabricator.services.mozilla.com/D109217

docshell/test/navigation/mochitest.ini

@@ -134,7 +134,6 @@ skip-if =
 [test_bug1326251.html]
 skip-if = toolkit == 'android' || sessionHistoryInParent # It relies on the bfcache
 [test_bug1379762.html]
-skip-if = sessionHistoryInParent # It relies on the bfcache
 [test_static_and_dynamic.html]
 skip-if = true # This was disabled for a few years now anyway, bug 1677544
 [test_sibling-matching-parent.html]

docshell/test/navigation/test_bug1379762.html

@@ -55,7 +55,10 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=
     SimpleTest.waitForExplicitFinish();
 
     function runTest() {
-      window.open("file_bug1379762-1.html", "", "width=360,height=480,noopener");
+      // If Fission is disabled, the pref is no-op.
+      SpecialPowers.pushPrefEnv({set: [["fission.bfcacheInParent", true]]}, () => {
+        window.open("file_bug1379762-1.html", "", "width=360,height=480,noopener");
+      });
     }
 
   </script>