test: move cgo demo into _testgo, and make links

_demo/cgofull/py.go

@@ -1,24 +0,0 @@
-package main
-
-/*
-#cgo pkg-config: python3-embed
-#include <Python.h>
-*/
-import "C"
-import "fmt"
-
-func Initialize() {
-	C.Py_Initialize()
-}
-
-func Finalize() {
-	C.Py_Finalize()
-}
-
-func Run(code string) error {
-	if C.PyRun_SimpleString(C.CString(code)) != 0 {
-		C.PyErr_Print()
-		return fmt.Errorf("failed to run code")
-	}
-	return nil
-}