test: fix flaky onbeforeunload tests (#19085)

spec/fixtures/api/close-beforeunload-empty-string.html

@@ -13,7 +13,7 @@
       return '';
     }
   }
-  window.close();
+  window.onload = () => window.close();
 </script>
 </body>
 </html>

spec/fixtures/api/close-beforeunload-false.html

@@ -12,7 +12,8 @@
       return false;
     }
   }
-  window.close();
+  // unload events don't get run unless load events have run.
+  window.onload = () => window.close()
 </script>
 </body>
 </html>

spec/fixtures/api/close-beforeunload-undefined.html

@@ -6,7 +6,7 @@
       require('electron').remote.getCurrentWindow().emit('onbeforeunload');
     }, 0);
   }
-  window.close();
+  window.onload = () => window.close();
 </script>
 </body>
 </html>

spec/fixtures/api/close.html

@@ -4,7 +4,7 @@
   window.addEventListener('unload', function (e) {
     require('fs').writeFileSync(__dirname + '/close', 'close');
   }, false);
-  window.close();
+  window.onload = () => window.close();
 </script>
 </body>
 </html>