[path] Fix root path mocking.

recipe_modules/path/examples/full.py

 # Copyright 2014 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 DEPS = [
   'path',
   'platform',
   'properties',
   'step',
 ]
@@ skipping 38 matching lines @@
 
   file_path = api.path['tmp_base'].join('new_file')
   abspath = api.path.abspath(file_path)
   api.path.assert_absolute(abspath)
 
   api.step('touch me', ['touch', api.path.abspath(file_path)])
   # Assert for testing that a file exists.
   api.path.mock_add_paths(file_path)
   assert api.path.exists(file_path)
 
+  # Assert that we can mock filesystem paths.
+  root_path = ('C:\\Windows' if api.platform.is_win else '/bin')
+  api.path.mock_add_paths(root_path)
+  assert api.path.exists(root_path)
+
   realpath = api.path.realpath(file_path)
   assert api.path.exists(realpath)
 
   home_path = api.path.join(api.path.expanduser('~'), 'file')
   api.step('touch my home', ['touch', home_path])
   api.path.mock_add_paths(home_path)
   assert api.path.exists(home_path)
 
   # can mock copy paths. See the file module to do this for real.
   copy1 = api.path['start_dir'].join('copy1')
@@ skipping 75 matching lines @@
     yield (api.test('%s_luci' % platform) +
            api.platform.name(platform) +
            api.properties(**{
               '$recipe_engine/path': {
                 'cache_dir': '/c',
                 'temp_dir': '/t',
                 'cleanup_dir': '/build.dead',
               },
            }) +
            api.path.exists(api.path['tmp_base']))