refactor: move issue #1370 regression tests to _demo/go

Move the three regression test cases from _demo/ to _demo/go/:
- issue1370_gotypes (go/types.Object test)
- issue1370_goast (go/ast.Expr test)
- issue1370_geometry (custom interface test)

Updated import path in geometry.go to reflect new location.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <51194195+luoliwoshang@users.noreply.github.com>

_demo/go/issue1370_geometry/geometry.go

@@ -0,0 +1,32 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/goplus/llgo/_demo/go/issue1370_geometry/geometry"
+)
+
+func main() {
+	rect := geometry.NewRectangle(5.0, 3.0)
+
+	err := geometry.RegisterShape(rect, 42)
+	if err != nil {
+		println("FAIL: RegisterShape returned error")
+		return
+	}
+
+	if rect.GetID() != 42 {
+		println("FAIL: ID not set correctly")
+		return
+	}
+
+	area := rect.Area()
+	fmt.Printf("Area: %.1f\n", area)
+
+	if area != 15.0 {
+		println("FAIL: Area calculation incorrect")
+		return
+	}
+
+	println("SUCCESS: Custom interface with private methods works correctly")
+}

_demo/go/issue1370_geometry/geometry/geometry.go

@@ -0,0 +1,26 @@
+package geometry
+
+type Shape interface {
+	Area() float64
+	validate() bool
+	setID(int)
+}
+
+type Rectangle struct {
+	Width, Height float64
+	id            int
+}
+
+func (r *Rectangle) Area() float64  { return r.Width * r.Height }
+func (r *Rectangle) validate() bool { return r.Width > 0 && r.Height > 0 }
+func (r *Rectangle) setID(id int)   { r.id = id }
+func (r *Rectangle) GetID() int     { return r.id }
+
+func NewRectangle(width, height float64) *Rectangle {
+	return &Rectangle{Width: width, Height: height}
+}
+
+func RegisterShape(s Shape, id int) error {
+	s.setID(id)
+	return nil
+}