zip: add a test for the previous >65k files fix

This surprisingly takes 30 seconds on my fast machine
so disabling by default. Need to optimize the Writer
at some point.

R=golang-dev, r, r
CC=golang-dev
https://golang.org/cl/4815048

src/pkg/archive/zip/zip_test.go

+// Copyright 2011 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.
+
+// Tests that involve both reading and writing.
+
+package zip
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"testing"
+)
+
+type stringReaderAt string
+
+func (s stringReaderAt) ReadAt(p []byte, off int64) (n int, err os.Error) {
+	if off >= int64(len(s)) {
+		return 0, os.EOF
+	}
+	n = copy(p, s[off:])
+	return
+}
+
+func TestOver65kFiles(t *testing.T) {
+	if testing.Short() {
+		t.Logf("slow test; skipping")
+		return
+	}
+	buf := new(bytes.Buffer)
+	w := NewWriter(buf)
+	const nFiles = (1 << 16) + 42
+	for i := 0; i < nFiles; i++ {
+		_, err := w.Create(fmt.Sprintf("%d.dat", i))
+		if err != nil {
+			t.Fatalf("creating file %d: %v", i, err)
+		}
+	}
+	if err := w.Close(); err != nil {
+		t.Fatalf("Writer.Close: %v", err)
+	}
+	rat := stringReaderAt(buf.String())
+	zr, err := NewReader(rat, int64(len(rat)))
+	if err != nil {
+		t.Fatalf("NewReader: %v", err)
+	}
+	if got := len(zr.File); got != nFiles {
+		t.Fatalf("File contains %d files, want %d", got, nFiles)
+	}
+	for i := 0; i < nFiles; i++ {
+		want := fmt.Sprintf("%d.dat", i)
+		if zr.File[i].Name != want {
+			t.Fatalf("File(%d) = %q, want %q", i, zr.File[i].Name, want)
+		}
+	}
+}