Assert the controller's buckets in the builder-on-the-bus fixture (issue 1945)

A merge the controller hears is now kept in the bucket of the merges (ADR 0539 §17); the test that sends one passed
only when an earlier test on the shared bus had made that bucket.

cmd/mesh-controller/builder_on_the_bus_test.go

@@ -47,6 +47,11 @@ func aBuilderOnTheBus(t *testing.T) builderOnTheBus {
 		}
 		*c.into = kv
 	}
+	// And the controller's own buckets, as its start asserts them: a merge it hears is kept in the bucket of the merges
+	// (ADR 0539 §17), which no other test need have made on this bus first.
+	if err := js.EnsureControllerBuckets(); err != nil {
+		t.Fatal(err)
+	}
 	servingBus.Store(js)
 	t.Cleanup(func() { servingBus.Store(nil) })
 	withTrunkList(t, true)