test: Fix secondary error thrown from unhandled errors (#7381)

In #7345, we disabled Jasmine's global error handler. However, jasmine
still tries to call this. So instead of replacing it with null, replace
it with a stub.

Also add comments about why we are able to do this and still handle
global errors in our own way.

test/test/boot.js

@@ -352,8 +352,10 @@ function configureJasmineEnvironment() {
     await loadNodeModules();
 
     // Replace jasmine's global error handler, since we have our own more
-    // nuanced version.
-    window.onerror = null;
+    // nuanced version.  You can't set this to null, since jasmine still tries
+    // to call it.  Note also that our handler uses window.addEventListener
+    // instead of window.onerror.
+    window.onerror = () => {};
   });
 
   const originalSetTimeout = window.setTimeout;