recipe engine: extract infra-specific paths out of the engine

recipe_modules/shutil/example.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from recipe_engine.types import freeze
 
 
 DEPS = [
   'path',
   'shutil',
@@ skipping 33 matching lines @@
     actual_content = api.shutil.read(
         'read_%s' % name, 'tmp_file.txt',
         test_data=content
     )
     msg = 'expected %s but got %s' % (content, actual_content)
     assert actual_content == content, msg
 
   try:
     # copytree
     content = 'some file content'
-    tmp_dir = api.path['slave_build'].join('copytree_example_tmp')
+    tmp_dir = api.path['tmp'].join('copytree_example_tmp')
     api.shutil.makedirs('makedirs', tmp_dir)
     path = tmp_dir.join('dummy_file')
     api.shutil.write('write %s' % path, path, content)
-    new_tmp = api.path['slave_build'].join('copytree_example_tmp2')
+    new_tmp = api.path['tmp'].join('copytree_example_tmp2')
     new_path = new_tmp.join('dummy_file')
     api.shutil.copytree('copytree', tmp_dir, new_tmp)
     actual_content = api.shutil.read('read %s' % new_path, new_path,
                                    test_data=content)
     assert actual_content == content
 
     # glob.
     files = api.shutil.glob(
         'glob', tmp_dir.join('*'),
         test_data=[tmp_dir.join('dummy_file')])
     assert files == [str(tmp_dir.join('dummy_file'))], files
 
   finally:
     api.shutil.rmtree(tmp_dir, name='cleanup')
     api.shutil.rmtree(new_tmp, name='cleanup2')
 
 
 def GenTests(api):
   yield api.test('basic')