test-web: Improve WebContent crash handling

Two fixes for crash handling:

1. Re-setup output capture after WebContent respawns. When WebContent
   crashes, a new process is spawned with new stdout/stderr pipes.
   We need to set up new notifiers to drain them, otherwise the new
   WebContent blocks on write() when its pipe buffer fills up.

2. Disconnect child view crash handlers between tests. Child views
   (from window.open, iframes, etc.) persist after a test completes.
   If they crash later, we don't want that to affect subsequent tests.

Tests/LibWeb/test-web/main.cpp

@@ -958,8 +958,14 @@ static void set_ui_callbacks_for_tests(TestWebView& view)
     };
 
     view.on_web_content_crashed = [&view]() {
-        if (auto test = s_test_by_view.get(&view); test.has_value())
+        // Re-setup output capture for the respawned WebContent process
+        // (handle_web_content_process_crash already ran and respawned it)
+        s_output_captures.remove(&view);
+        setup_output_capture_for_view(view);
+
+        if (auto test = s_test_by_view.get(&view); test.has_value()) {
             view.on_test_complete({ *test.value(), TestResult::Crashed });
+        }
     };
 }
 
@@ -1139,6 +1145,9 @@ static ErrorOr<int> run_tests(Core::AnonymousBuffer const& theme, Web::DevicePix
             view->on_reference_test_metadata = {};
             view->on_set_test_timeout = {};
 
+            // Disconnect child crash handlers so old child crashes don't affect the next test
+            view->disconnect_child_crash_handlers();
+
             // Don't try to reset zoom if WebContent crashed - it's gone
             if (result.result != TestResult::Crashed)
                 view->reset_zoom();