test: Adjust hack in TestGrafanaClient for non-crashing page (#19667)

Later Grafana versions [1] fixed the page crash on "Failed to fetch plugins
from catalog", and just log it to the console now. That will make the
"wait for false" loop timeout and eventually fail. If that happens, then
all is actually well.

[1] cockpit-project/bots#5601

test/verify/check-metrics

@@ -1525,6 +1525,10 @@ class TestGrafanaClient(testlib.MachineCase):
             except RuntimeError as e:
                 if "Failed to fetch plugins from catalog" not in str(e):
                     raise
+            except testlib.Error as e:
+                if not e.msg.startswith("timeout"):
+                    raise
+                # no plugin check error? great!
 
             # Grafana auto-discovers "host" variable for incoming metrics; it takes a while to receive the first
             # measurement; that event is not observable directly in Grafana, and the dashboard does not auto-update to