bug#21743: Allow unloading single features for tests

[Andreas Röhler]

 GNU Emacs 24.4.1 (i586-pc-linux-gnu, GTK+ Version 3.14.5) of 
2015-03-07 on binet, modified by Debian

Whilest python-mode.el and python.el in general don't conflict, when 
testing python-mode.el unloading the built-in python.el seems preferable.

Unfortunatly, with CEDET loadet, get the following:

error: "Loaded libraries 
(\"/usr/share/emacs/24.4/lisp/cedet/semantic/wisent/python.elc\") depend 
on /usr/share/emacs/24.4/lisp/progmodes/python.elc"
Aborted: Ran 1 tests, 0 results were as expected

Suggest not to hard-code this case as error, rather issue a warning.

BTW resides another python.el inside CEDET, which doesn't look optimal.

Thanks all,

Andreas