test: add test of deferred delete

Updates #19710

Change-Id: I37d19a4a02b9010cb5f9062b3d141d5d65e12e01
Reviewed-on: https://go-review.googlesource.com/43497
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue19710.go

+// run
+
+// Copyright 2017 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 19710: mishandled defer delete(...)
+
+package main
+
+func main() {
+	if n := len(f()); n != 0 {
+		println("got", n, "want 0")
+		panic("bad defer delete")
+	}
+}
+
+func f() map[int]bool {
+	m := map[int]bool{}
+	for i := 0; i < 3; i++ {
+		m[i] = true
+		defer delete(m, i)
+	}
+	return m
+}