[path] Fix root path mocking.

README.recipes.md

 <!--- AUTOGENERATED BY `./recipes.py test train` -->
 # Package documentation for [recipe\_engine](https://github.com/luci/recipes-py)
 ## Table of Contents
 
 **[Recipe Modules](#recipe-modules)**
   * [context](#recipe_modules--context) &mdash; The context module provides APIs for manipulating a few pieces of 'ambient' data that affect how steps are run.
   * [file](#recipe_modules--file) &mdash; File manipulation (read/write/delete/glob) methods.
   * [generator_script](#recipe_modules--generator_script) &mdash; A simplistic method for running steps generated by an external script.
   * [json](#recipe_modules--json) &mdash; Methods for producing and consuming JSON.
   * [path](#recipe_modules--path) &mdash; All functions related to manipulating paths in recipes.
@@ skipping 489 matching lines @@
   * `api.path['checkout']` - This directory is set by various 'checkout' modules
     in recipes. It was originally intended to make recipes easier to read and
     make code somewhat generic or homogenous, but this was a mistake. New code
     should avoid 'checkout', and instead just explicitly pass paths around. This
     path may be removed in the future.
 
 There are other anchor points which can be defined (e.g. by the
 `depot_tools/infra_paths` module). Refer to those modules for additional
 documentation.
 
-#### **class [PathApi](/recipe_modules/path/api.py#L196)([RecipeApi](/recipe_engine/recipe_api.py#L884)):**
+#### **class [PathApi](/recipe_modules/path/api.py#L197)([RecipeApi](/recipe_engine/recipe_api.py#L884)):**
 
-&mdash; **def [\_\_getitem\_\_](/recipe_modules/path/api.py#L421)(self, name):**
+&mdash; **def [\_\_getitem\_\_](/recipe_modules/path/api.py#L422)(self, name):**
 
 Gets the base path named `name`. See module docstring for more
 information.
 
-&mdash; **def [abs\_to\_path](/recipe_modules/path/api.py#L351)(self, abs_string_path):**
+&mdash; **def [abs\_to\_path](/recipe_modules/path/api.py#L352)(self, abs_string_path):**
 
 Converts an absolute path string `string_path` to a real Path object,
 using the most appropriate known base path.
 
   * abs_string_path MUST be an absolute path
   * abs_string_path MUST be rooted in one of the configured base paths known
     to the path module.
 
 This method will find the longest match in all the following:
   * module resource paths
   * recipe resource paths
   * package repo paths
   * dynamic_paths
   * base_paths
 
 Example:
 ```
 # assume [START_DIR] == "/basis/dir/for/recipe"
 api.path.abs_to_path("/basis/dir/for/recipe/some/other/dir") ->
   Path("[START_DIR]/some/other/dir")
 ```
 
 Raises an ValueError if the preconditions are not met, otherwise returns the
 Path object.
 
-&mdash; **def [assert\_absolute](/recipe_modules/path/api.py#L326)(self, path):**
+&mdash; **def [assert\_absolute](/recipe_modules/path/api.py#L327)(self, path):**
 
 Raises AssertionError if the given path is not an absolute path.
 
 Args:
   * path (Path|str) - The path to check.
 
-&mdash; **def [get](/recipe_modules/path/api.py#L414)(self, name, default=None):**
+&mdash; **def [get](/recipe_modules/path/api.py#L415)(self, name, default=None):**
 
 Gets the base path named `name`. See module docstring for more
 information.
 
-&mdash; **def [get\_config\_defaults](/recipe_modules/path/api.py#L208)(self):**
+&mdash; **def [get\_config\_defaults](/recipe_modules/path/api.py#L209)(self):**
 
 Internal recipe implementation function.
 
-&mdash; **def [initialize](/recipe_modules/path/api.py#L270)(self):**
+&mdash; **def [initialize](/recipe_modules/path/api.py#L271)(self):**
 
 Internal recipe implementation function.
 
-&mdash; **def [mkdtemp](/recipe_modules/path/api.py#L334)(self, prefix):**
+&mdash; **def [mkdtemp](/recipe_modules/path/api.py#L335)(self, prefix):**
 
 Makes a new temporary directory, returns Path to it.
 
-&mdash; **def [mock\_add\_paths](/recipe_modules/path/api.py#L305)(self, path):**
+&mdash; **def [mock\_add\_paths](/recipe_modules/path/api.py#L306)(self, path):**
 
 For testing purposes, mark that |path| exists.
 
-&mdash; **def [mock\_copy\_paths](/recipe_modules/path/api.py#L310)(self, source, dest):**
+&mdash; **def [mock\_copy\_paths](/recipe_modules/path/api.py#L311)(self, source, dest):**
 
 For testing purposes, copy |source| to |dest|.
 
-&mdash; **def [mock\_remove\_paths](/recipe_modules/path/api.py#L315)(self, path, filt=(lambda p: True)):**
+&mdash; **def [mock\_remove\_paths](/recipe_modules/path/api.py#L316)(self, path, filt=(lambda p: True)):**
 
 For testing purposes, assert that |path| doesn't exist.
 
 Args:
   * path (str|Path) - The path to remove.
   * filt (func[str] bool) - Called for every candidate path. Return
     True to remove this path.
 ### *recipe_modules* / [platform](/recipe_modules/platform)
 
 Mockable system platform identity functions.
@@ skipping 741 matching lines @@
 
 [DEPS](/recipe_modules/url/tests/join.py#L5): [step](#recipe_modules--step), [url](#recipe_modules--url)
 
 &mdash; **def [RunSteps](/recipe_modules/url/tests/join.py#L11)(api):**
 ### *recipes* / [url:tests/validate\_url](/recipe_modules/url/tests/validate_url.py)
 
 [DEPS](/recipe_modules/url/tests/validate_url.py#L5): [properties](#recipe_modules--properties), [step](#recipe_modules--step), [url](#recipe_modules--url)
 
 &mdash; **def [RunSteps](/recipe_modules/url/tests/validate_url.py#L12)(api):**