test: move cgo demo into _testgo, and make links

cl/_testgo/cgocfiles/cgocfiles.go

@@ -0,0 +1,15 @@
+package main
+
+/*
+#include "in.h"
+*/
+import "C"
+import "fmt"
+
+func main() {
+	r := C.test_structs(&C.s4{a: 1}, &C.s8{a: 1, b: 2}, &C.s12{a: 1, b: 2, c: 3}, &C.s16{a: 1, b: 2, c: 3, d: 4}, &C.s20{a: 1, b: 2, c: 3, d: 4, e: 5})
+	fmt.Println(r)
+	if r != 35 {
+		panic("test_structs failed")
+	}
+}