test: Robustify Browser.assert_no_oops

Webdriver's script preloading doesn't happen for empty frames. So our
test-functions.js isn't available in tests which never navigate to a
page, such as TestConnection.

Do the first check with a direct `getElementById()` instead of relying
on our page helpers.

test/common/testlib.py

@@ -1366,7 +1366,7 @@ def assert_no_oops(self) -> None:
 
         if self.cdp and self.cdp.valid:
             self.switch_to_top()
-            if self.is_present("#navbar-oops"):
+            if self.eval_js("document.getElementById('navbar-oops')"):
                 assert not self.is_visible("#navbar-oops"), "Cockpit shows an Oops"