Avoid unneccessary qualification of tmp_filename in doctests.

Volker Braun pointed this out in comment:6:ticket:16645.
Trac #16645: Restore displaying animations in notebook

#12827 broke display of animations, and while #16608 restored that
functionality for `Animation.show()`, it remains broken for methods
`Animation.gif()` and `Animation.ffmpeg()`. Based on #15515, a more
complete fix is possible.

URL: http://trac.sagemath.org/16645
Reported by: gagern
Ticket author(s): Martin von Gagern
Reviewer(s): Volker Braun