Trac #3912: sage -t foo.tex should also test listings blocks not only verbatim

sage -t can check stuff in
{{{
\begin{verbatim}
sage: 2 + 3
5
\end{verbatim}
}}}

I propose to also support
{{{
\begin{lstlisting}
sage: 2 + 3
5
\end{lstlisting}
}}}
which looks so much nicer.

URL: http://trac.sagemath.org/3912
Reported by: malb
Ticket author(s): Frédéric Chapoton
Reviewer(s): André Apitzsch