[fetch] add tests for fetch subcommand parsing.

recipe_engine/fetch.py

Index: recipe_engine/fetch.py
diff --git a/recipe_engine/fetch.py b/recipe_engine/fetch.py
index dbfbc916eb4303f8a02e8aa35589eefa115a4b53..dab030a19ed2d453cf2f9b5cd98be0efc0af5bc8 100644
--- a/recipe_engine/fetch.py
+++ b/recipe_engine/fetch.py
@@ -602,7 +602,7 @@ def add_subparser(parser):
 
   def postprocess_func(parser, args):
     if args.no_fetch:
-      parser.error('--no-fetch doesn\'t make sense with fetch command')
+      parser.error('--no-fetch does not make sense with fetch command')
 
   fetch_p.set_defaults(
     command='fetch',