[recipes.py] move run arg parsing to its module.

recipe_modules/path/example.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 49 matching lines @@
     api.path['start_dir'].join('some', 'thing'),
     api.path['start_dir'],
     api.path.resource("module_resource.py"),
     api.path.resource(),
     api.resource("recipe_resource.py"),
     api.resource(),
   ]
   for p in paths_to_convert:
     after = api.path.abs_to_path(str(p))
     api.step('converted path %s' % p, ['echo', after])
-    assert after == p, p
+    assert after == p, (after, p)
 
   try:
     api.path.abs_to_path('non/../absolute')
     assert False, "this should have thrown"  # pragma: no cover
   except ValueError as ex:
     assert "is not absolute" in ex.message, ex
 
   try:
     if api.platform.is_win:
       api.path.abs_to_path('C:\some\other\root\non\absolute')
@@ skipping 24 matching lines @@
 
     yield (api.test('%s_luci' % platform) +
            api.platform.name(platform) +
            api.properties(**{
               '$recipe_engine/path': {
                 'cache_dir': '/c',
                 'tmp_dir': '/t'
               },
            }) +
            api.path.exists(api.path['tmp_base']))