lib/model: Improve filesystem operations during tests (fixes #5422)

* lib/fs, lib/model: Improve filesystem operations during tests (fixes #5422)

Introduces MustFilesystem that panics on errors and should be used for operations
during testing which must never fail.
Create temporary directories outside of testdata.

* don't do a filesystem, just a wrapper around os for testing

* fix copyright

lib/model/testos_test.go

@@ -0,0 +1,82 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package model
+
+import (
+	"os"
+	"time"
+)
+
+// fatal is the required common interface between *testing.B and *testing.T
+type fatal interface {
+	Fatal(...interface{})
+}
+
+type fatalOs struct {
+	fatal
+}
+
+func (f *fatalOs) must(fn func() error) {
+	if err := fn(); err != nil {
+		f.Fatal(err)
+	}
+}
+
+func (f *fatalOs) Chmod(name string, mode os.FileMode) error {
+	f.must(func() error { return os.Chmod(name, mode) })
+	return nil
+}
+
+func (f *fatalOs) Chtimes(name string, atime time.Time, mtime time.Time) error {
+	f.must(func() error { return os.Chtimes(name, atime, mtime) })
+	return nil
+}
+
+func (f *fatalOs) Create(name string) (*os.File, error) {
+	file, err := os.Create(name)
+	if err != nil {
+		f.Fatal(err)
+	}
+	return file, nil
+}
+
+func (f *fatalOs) Mkdir(name string, perm os.FileMode) error {
+	f.must(func() error { return os.Mkdir(name, perm) })
+	return nil
+}
+
+func (f *fatalOs) MkdirAll(name string, perm os.FileMode) error {
+	f.must(func() error { return os.MkdirAll(name, perm) })
+	return nil
+}
+
+func (f *fatalOs) Remove(name string) error {
+	if err := os.Remove(name); err != nil && !os.IsNotExist(err) {
+		f.Fatal(err)
+	}
+	return nil
+}
+
+func (f *fatalOs) RemoveAll(name string) error {
+	if err := os.RemoveAll(name); err != nil && !os.IsNotExist(err) {
+		f.Fatal(err)
+	}
+	return nil
+}
+
+func (f *fatalOs) Rename(oldname, newname string) error {
+	f.must(func() error { return os.Rename(oldname, newname) })
+	return nil
+}
+
+func (f *fatalOs) Stat(name string) (os.FileInfo, error) {
+	info, err := os.Stat(name)
+	if err != nil {
+		f.Fatal(err)
+	}
+	return info, nil
+}