test: add test that gofrontend fails

For #52870

Change-Id: Ic0791af4283c9e426f7cbfab0514517ff84cfa80
Reviewed-on: https://go-review.googlesource.com/c/go/+/412535
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

test/fixedbugs/issue52870.go

@@ -0,0 +1,27 @@
+// compile
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 52870: gofrontend gave incorrect error when incorrectly
+// compiling ambiguous promoted method.
+
+package p
+
+type S1 struct {
+	*S2
+}
+
+type S2 struct {
+	T3
+	T4
+}
+
+type T3 int32
+
+func (T3) M() {}
+
+type T4 int32
+
+func (T4) M() {}