swarm/api: make api.NewManifest synchronous (#14880)

Previously, NewManifest was asynchronous so subsequent code which tried
to use the returned manifest could error as the manifest was not yet
persisted.

cmd/swarm/run_test.go

@@ -161,6 +161,7 @@ func newTestNode(t *testing.T, dir string) *testNode {
 	conf := &node.Config{
 		DataDir: dir,
 		IPCPath: "bzzd.ipc",
+		NoUSB:   true,
 	}
 	n, err := node.New(conf)
 	if err != nil {