Improve testing by comparing behavior of subprocess to subprocess2

This makes sure no regression relative to subprocess is introduced in
subprocess2.

R=dpranke@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/8722001

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@112217 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/subprocess2.py b/subprocess2.py
index 8964aad..e65186e 100644
--- a/subprocess2.py
+++ b/subprocess2.py
@@ -310,4 +310,6 @@
   - As per doc, "The stdout argument is not allowed as it is used internally."
   """
   kwargs.setdefault('stdin', VOID)
+  if 'stdout' in kwargs:
+    raise ValueError('stdout argument not allowed, it will be overridden.')
   return check_call_out(args, stdout=PIPE, **kwargs)[0]