test: add regression tests for issue #1370

Add three regression test cases for cross-package interface private method calls:

1. _cmptest/issue1370_gotypes/ - Tests go/types.Object interface with Scope.Insert()
   which calls the private setParent() method

2. _cmptest/issue1370_goast/ - Tests go/ast.Expr interface conversion with ast.Ident
   which has a private exprNode() method

3. _cmptest/issue1370_geometry/ - Tests custom geometry.Shape interface with private
   methods (validate, setID) across package boundaries

These tests verify the fix in ssa/abitype.go correctly sets the interface metadata's
PkgPath to the interface definition package instead of the compilation package,
preventing segmentation faults when calling private methods.

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

_cmptest/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")
+}