Join 11 (try 2): the chunked-answer tests flush the holder's verb before another connection asks it

The tests of A-249 and A-257 served the verb on one connection and asked it from another at once; the
subscription was still in the holder's buffer when the ask reached the bus, so the ask was answered 503
no responders (13 of 300 runs with -cpu 1 before, 0 of 2000 after). The runner flushes after serving
(runtime.go:352, :807); the tests now do as it does.

Builds: design 47 G A-249, A-257
Claude-Session: https://claude.ai/code/session_01NcbgfGKaxw984XyAPfLm4h

node-tools/internal/bus/chunked_test.go

@@ -57,6 +57,7 @@ func TestAChunkedSeatAnswerIsReceivedByTheRunnerAndHandedWhole(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer stop()
+	holder.Flush() // the bus holds the verb's subscription before it is asked: the asker is another connection
 	got, err := asker.AskAsChunked("mesh-build-go", "seat:mesh-catalog.input", map[string]any{"digest": "d1"})
 	if err != nil {
 		t.Fatal(err)
@@ -93,6 +94,7 @@ func TestABundlesChunkedAnswerIsSentByTheRunner(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer stop()
+	holder.Flush() // the bus holds the verb's subscription before it is asked: the asker is another connection
 	got, err := asker.AskAsChunked("mesh-delivery-indexer", "seat:node-build-go.output@anchor", map[string]any{"ask": "a1"})
 	if err != nil {
 		t.Fatal(err)