[context] Add path prefix/suffix manipulation.

recipe_modules/context/tests/env.py

 # Copyright 2017 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 = [
   "context",
   "path",
   "step",
 ]
 
 
 def RunSteps(api):
   api.step('no env', ['echo', 'hello'])
 
   with api.context(env={"SOMETHING": "1"}):
     api.step('with env', ['echo', 'hello'])
 
     with api.context(env={"SOMETHING_ELSE": "0"}):
       api.step('with 2 envs', ['echo', 'hello'])
 
+  with api.context(env={'FOO': 'bar'}):
+    api.step('env step', ['bash', '-c', 'echo $FOO'])
+
+    base_path = 'foo%s%%(FOO)s%sbaz' % (api.path.pathsep, api.path.pathsep)
+    with api.context(env={'FOO': base_path}):
+      api.step('env step augmented', ['bash', '-c', 'echo $FOO'])
+
+      pants = api.path['start_dir'].join('pants')
+      shirt = api.path['start_dir'].join('shirt')
+      with api.context(env={'FOO': api.context.Prefix(pants, shirt)}):
+        api.step('env step with prefix', ['bash', '-c', 'echo $FOO'])
+
+  # Can set the path of default environment variables.
+  with api.context(env={'FOO': api.context.Prefix(shirt)}):
+    api.step('env with default value',
+             ['bash', '-c', 'echo $FOO'])
+
+
 def GenTests(api):
   yield api.test('basic')