chat: gosh: switch from Logf/Fatalf to TB, and related

This change makes most of the changes described in the final
proposal in https://v.io/i/1128 - namely, it makes all the
requisite API changes.

Things not done in this change:
- "Good PC" logging
- Printing Cmd stdout/stderr on failure

Those will be done in subsequent changes.

MultiPart: 7/9

Change-Id: I576f68de542ac2ed43a0ca0dd81a38bff2529efe
diff --git a/clients/shell/go/src/v.io/x/chat/channel_test.go b/clients/shell/go/src/v.io/x/chat/channel_test.go
index 63ccbc2..9800320 100644
--- a/clients/shell/go/src/v.io/x/chat/channel_test.go
+++ b/clients/shell/go/src/v.io/x/chat/channel_test.go
@@ -84,7 +84,7 @@
 }
 
 func TestMembers(t *testing.T) {
-	sh := v23test.NewShell(t, v23test.Opts{})
+	sh := v23test.NewShell(t, nil)
 	defer sh.Cleanup()
 	ctx := sh.Ctx
 
@@ -154,7 +154,7 @@
 }
 
 func TestBroadcastMessage(t *testing.T) {
-	sh := v23test.NewShell(t, v23test.Opts{})
+	sh := v23test.NewShell(t, nil)
 	defer sh.Cleanup()
 	ctx := sh.Ctx