veyron/lib/modules: Use the agent for managing credentials in modules.Shell.

Change-Id: I17737af8d4fe2281d2f0e5a267cbff268758f788
diff --git a/security/flag/flag_test.go b/security/flag/flag_test.go
index f2edf84..fbf46bc 100644
--- a/security/flag/flag_test.go
+++ b/security/flag/flag_test.go
@@ -67,7 +67,7 @@
 }
 
 func TestNewAuthorizerOrDie(t *testing.T) {
-	sh, err := modules.NewShell(nil)
+	sh, err := modules.NewShell(nil, nil)
 	if err != nil {
 		t.Fatalf("unexpected error: %s", err)
 	}