First cut of testing infrastructure for recipes.

scripts/slave/recipe_util.py

Index: scripts/slave/recipe_util.py
diff --git a/scripts/slave/recipe_util.py b/scripts/slave/recipe_util.py
index a67cd054e54e282c524e3693c82009247c347810..e3823e68266d1dc271d53790b98aee03bf4f1072 100644
--- a/scripts/slave/recipe_util.py
+++ b/scripts/slave/recipe_util.py
@@ -4,6 +4,7 @@
 
 """This module holds utilities which make writing recipes easier."""
 
+import contextlib as _contextlib
 import os as _os
 
 # e.g. /b/build/slave/<slave-name>/build
@@ -57,6 +58,32 @@ functionality provided by annotated_run.
 checkout_path = _path_method('checkout_path', "%(CheckoutRootPlaceholder)s")
 
 
+@_contextlib.contextmanager
+def mock_paths():
+  path_base_names = ['depot_tools', 'build_internal', 'build', 'slave_build',
+                     'root']
+  g = globals()
+  tokens = {}
+  path_funcs = {}
+  try:
+    for name in path_base_names:
+      token_name = (name+"_root").upper()
+      token_val = '[%s]' % token_name
+      path_func_name = (name+"_path")
+
+      if token_name in g:
+        tokens[token_name] = g[token_name]
+        g[token_name] = token_val
+
+      if path_func_name in g:
+        path_funcs[path_func_name] = g[path_func_name]
+        g[path_func_name] = _path_method(path_func_name, token_val)
+    yield
+  finally:
+    g.update(tokens)
+    g.update(path_funcs)
+
+
 def deep_set(obj, key_vals):
   """Take an object (a dict or list), and a list of key/value pairs to set,
   and transform it by replacing items in obj at the key locations with the