fix race in test setup/teardown

not easily triggered, but it happened just now on a build server.

imapserver/fuzz_test.go

@@ -71,8 +71,7 @@ func FuzzServer(f *testing.F) {
 	if err != nil {
 		f.Fatalf("set password: %v", err)
 	}
-	done := store.Switchboard()
-	defer close(done)
+	defer store.Switchboard()()
 
 	comm := store.RegisterComm(acc)
 	defer comm.Unregister()

imapserver/server_test.go

@@ -346,11 +346,9 @@ func startArgs(t *testing.T, first, isTLS, allowLoginWithoutTLS bool) *testconn
 		err = acc.SetPassword("testtest")
 		tcheck(t, err, "set password")
 	}
-	var switchDone chan struct{}
+	switchStop := func() {}
 	if first {
-		switchDone = store.Switchboard()
-	} else {
-		switchDone = make(chan struct{}) // Dummy, that can be closed.
+		switchStop = store.Switchboard()
 	}
 
 	serverConn, clientConn := net.Pipe()
@@ -368,7 +366,7 @@ func startArgs(t *testing.T, first, isTLS, allowLoginWithoutTLS bool) *testconn
 	cid := connCounter
 	go func() {
 		serve("test", cid, tlsConfig, serverConn, isTLS, allowLoginWithoutTLS)
-		close(switchDone)
+		switchStop()
 		close(done)
 	}()
 	client, err := imapclient.New(clientConn, true)