cmd/cloner: add regression test for slice nil/empty semantics

We had a misstep with the semantics when applying an optimization that
showed up in the roll into corp. This test ensures that case and related
cases must be retained.

Updates #9410
Updates #9601
Signed-off-by: James Tucker <james@tailscale.com>

cmd/cloner/cloner_test.go

@@ -0,0 +1,60 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+package main
+
+import (
+	"reflect"
+	"testing"
+
+	"tailscale.com/cmd/cloner/clonerex"
+)
+
+func TestSliceContainer(t *testing.T) {
+	num := 5
+	examples := []struct {
+		name string
+		in   *clonerex.SliceContianer
+	}{
+		{
+			name: "nil",
+			in:   nil,
+		},
+		{
+			name: "zero",
+			in:   &clonerex.SliceContianer{},
+		},
+		{
+			name: "empty",
+			in: &clonerex.SliceContianer{
+				Slice: []*int{},
+			},
+		},
+		{
+			name: "nils",
+			in: &clonerex.SliceContianer{
+				Slice: []*int{nil, nil, nil, nil, nil},
+			},
+		},
+		{
+			name: "one",
+			in: &clonerex.SliceContianer{
+				Slice: []*int{&num},
+			},
+		},
+		{
+			name: "several",
+			in: &clonerex.SliceContianer{
+				Slice: []*int{&num, &num, &num, &num, &num},
+			},
+		},
+	}
+
+	for _, ex := range examples {
+		t.Run(ex.name, func(t *testing.T) {
+			out := ex.in.Clone()
+			if !reflect.DeepEqual(ex.in, out) {
+				t.Errorf("Clone() = %v, want %v", out, ex.in)
+			}
+		})
+	}
+}