refactor: move issue #1370 regression tests from _cmptest to _demo

Move the three regression test cases for issue #1370 from _cmptest/ to _demo/:
- 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.

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Co-authored-by: luoliwoshang <51194195+luoliwoshang@users.noreply.github.com>

_demo/issue1370_geometry/geometry.go

@@ -0,0 +1,32 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/goplus/llgo/_demo/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")
+}