Scope the node-type fixture check to the built-ins

Installing the connectors made this fail, which is the test working: it
insists every offered type is exercised. But a connector is a separate
package with its own tests, so it is not this suite's to cover.

backend/tests/flow/test_node_types.py

@@ -125,6 +125,11 @@ def test_mqtt_routes_topics_by_port():
 
 
 def test_every_offered_type_has_a_fixture():
-    # A new node type without a fixture here would ship untested.
-    offered = {info.type for info in node_type_info() if info.type != "python"}
+    # A new built-in without a fixture here would ship untested. Connectors are
+    # separate packages and carry their own tests, so they are not this suite's.
+    offered = {
+        info.type
+        for info in node_type_info()
+        if info.type != "python" and not info.plugin
+    }
     assert offered == set(FIXTURES)