Make tests not create std{out,err}_logfile in cwd

[msabramo]

by modifying 3 tests to set instance.std{out,err}_logfile to a filename in the temp directory by using tempfile.mktemp

This is an alternative solution to the issue mentioned in #382 and #377.

Before:

$ /bin/rm std*_logfile
$ ls -l std*_logfile
zsh: no matches found: std*_logfile
$ tox -e py26
...
$ ls -l std*_logfile
-rw-r--r--  1 marca  staff     0B Apr 14 08:29 stderr_logfile
-rw-r--r--  1 marca  staff     0B Apr 14 08:29 stdout_logfile

After:

$ /bin/rm std*_logfile
$ ls -l std*_logfile
zsh: no matches found: std*_logfile
$ tox -e py26
...
$ ls -l std*_logfile
zsh: no matches found: std*_logfile

Cc: @mnaberez, @mcdonc, @gcarothers, @graffic

[msabramo]

I think that #418 is better because it takes care to delete the temp files.

I'll close this...