test(demo): add regression demos for issue #1370 cases

- case1: go/types.Object with Scope.Insert() calling private setParent()
- case2: go/ast.Expr interface conversion with private exprNode()

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

_demo/go/issue1370_case1/main.go

@@ -0,0 +1,18 @@
+package main
+
+import (
+	"go/types"
+)
+
+func main() {
+	scope := types.NewScope(nil, 0, 0, "test")
+	obj := types.NewVar(0, nil, "x", types.Typ[types.Int])
+
+	scope.Insert(obj)
+
+	if scope.Lookup("x") == obj {
+		println("SUCCESS: Scope.Insert and Lookup work correctly")
+	} else {
+		println("FAIL: Lookup returned wrong object")
+	}
+}