testlib: Force page loading with Chromium 119

The same "browser optimizes away page loads" behaviour that we fixed for
Firefox in commit 49ee017 now also affects Chromium 119.
Unfortunately that does not have a working `Page.navigate` option to
force a page load. So force it by navigating to the blank page first.

test/common/testlib.py

@@ -241,6 +241,11 @@ def open(self, href: str, cookie: Optional[Dict[str, str]] = None, tls: bool = F
             # Force a reload in this case, to make tests and the waitPageLoad below predictable
             # But that option has the inverse effect with Chromium (argh)
             opts["transitionType"] = "reload"
+        elif self.cdp.browser.name == 'chromium':
+            # Chromium also optimizes this away, but doesn't have a knob to force loading
+            # so load the blank page first
+            self.cdp.invoke("Page.navigate", url="about:blank")
+            self.cdp.invoke("waitPageLoad", timeout=5)
         self.cdp.invoke("Page.navigate", url=href, **opts)
         self.cdp.invoke("waitPageLoad", timeout=self.cdp.timeout)