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

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