backend-test-utils: fix handling of root scoped services

Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Co-authored-by: Johan Haals <johan.haals@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/curvy-lemons-change.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-test-utils': patch
+---
+
+Fixed handling of root scoped services in `startTestBackend`.

packages/backend-test-utils/src/next/wiring/TestBackend.ts

@@ -63,10 +63,18 @@ export async function startTestBackend<
     if (Array.isArray(serviceDef)) {
       // if type is ExtensionPoint?
       // do something differently?
+      const [ref, impl] = serviceDef;
+      if (ref.scope === 'plugin') {
+        return createServiceFactory({
+          service: ref,
+          deps: {},
+          factory: async () => async () => impl,
+        });
+      }
       return createServiceFactory({
-        service: serviceDef[0],
+        service: ref,
         deps: {},
-        factory: async () => async () => serviceDef[1],
+        factory: async () => impl,
       });
     }
     return serviceDef as ServiceFactory;