depot_tools: add test_api to infra_paths recipe module

recipe_modules/infra_paths/test_api.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 import recipe_test_api
+from recipe_engine.config_types import Path, NamedBasePath
+
+class InfraPathsTestApi(recipe_test_api.RecipeTestApi):
+  @recipe_test_api.mod_test_data
+  @staticmethod
+  def exists(*paths):
+    assert all(isinstance(p, Path) for p in paths)
+    return paths
+
+  def __getitem__(self, name):
+    return Path(NamedBasePath(name))