Trigger the DOMContentLoaded on the Window

This is hacky, but it's inspired by how NetSurf does it. While the Window isn't
the parent of the Document, many events should bubble from the Document to the
Window. libdom simply doesn't handle this (it has no concept of a Window, and
the Document has no parent).

We potentially need to do this for multiple event types (NetSurf only does it
for the 'load' event as far as I can tell). It would be nice to find a generic
way to do this...maybe intercept any addEventListener on the body and
registering special events on the Window? For now, `DOMContentLoaded` is the
blocking (for finance.yahoo.com) and we can see if this is really an issue for
other event types.

src/testing.zig

@@ -474,6 +474,12 @@ pub const JsRunner = struct {
             return err;
         };
     }
+
+    pub fn dispatchDOMContentLoaded(self: *JsRunner) !void {
+        const HTMLDocument = @import("browser/html/document.zig").HTMLDocument;
+        const html_doc = self.page.window.document;
+        try HTMLDocument.documentIsLoaded(html_doc, self.page);
+    }
 };
 
 const RunnerOpts = struct {