veyron/security/agent: Use already generated TestMain instead of
TestHelperProcess.

Now only 4 uses of TestHelperProcess/DispatchInTest remain, but removing
these seem to be a bit more involved.

Change-Id: I0b7511cb60876ea4ca7c892d856b452783660e06
diff --git a/security/agent/agent_test.go b/security/agent/agent_test.go
index 9237f50..b2061dd 100644
--- a/security/agent/agent_test.go
+++ b/security/agent/agent_test.go
@@ -23,13 +23,7 @@
 	"v.io/core/veyron2/security"
 )
 
-func init() {
-	modules.RegisterChild("hang", "", getPrincipalAndHang)
-}
-
-func TestHelperProcess(t *testing.T) {
-	modules.DispatchInTest()
-}
+//go:generate v23 test generate
 
 func getPrincipalAndHang(stdin io.Reader, stdout, stderr io.Writer, env map[string]string, args ...string) error {
 	ctx, shutdown := testutil.InitForTest()
@@ -153,7 +147,7 @@
 		t.Fatal(err)
 	}
 	// The child process will connect to the agent
-	h, err := sh.Start("hang", nil)
+	h, err := sh.Start("getPrincipalAndHang", nil)
 	if err != nil {
 		t.Fatal(err)
 	}