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_goast/goast.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"fmt"
+	"go/ast"
+	"go/token"
+)
+
+func main() {
+	ident := &ast.Ident{
+		NamePos: token.Pos(42),
+		Name:    "foo",
+	}
+
+	var expr ast.Expr = ident
+	pos := expr.Pos()
+
+	fmt.Printf("Position: %d\n", pos)
+	if pos != 42 {
+		println("FAIL: Position mismatch")
+		return
+	}
+
+	println("SUCCESS: ast.Expr interface method calls work correctly")
+}