Clean up test files, no substantive changes.

These changes prepare for the changes to the tests that are coming, but
make no change to the behavior themselves.
Add two tests of problems-with-files

One test passes already, the other fails.  Fix in next commit.
Properly convert files from Webob to pure files.

Webob represents uploaded files as cgi.FieldStorage objects.  The
XModule code expects pure Python file objects.  Each FieldStorage object
is wrapped to present the proper file interface, with file names.