TestProcessConfig.tearDown: Cleanup std{err,out}_logfile files if they exist

[msabramo]

This appears to fix the problem of these files being left around after test runs, alluded to in #377 (comment)

[msabramo]

This seems to prevent these files from being left around.

$ git log -n1
commit f01e17eae77e317221cb90578b23333ebf356b90
Author: Marc Abramowitz <marc@marc-abramowitz.com>
Date:   Sat Feb 22 14:15:02 2014 -0800

    TestProcessConfig.tearDown: Cleanup std{err,out}_logfile files if they
    exist

$ find . -name 'std*_logfile'

$ tox -e py26
...
Ran 758 tests in 3.135s

OK
...
  py26: commands succeeded
  congratulations :)

$ find . -name 'std*_logfile'

Heck, it'll even nuke those files if they already exist!

$ touch stdout_logfile stderr_logfile

$ find . -name 'std*_logfile'
./stderr_logfile
./stdout_logfile

$ tox -e py26
...
  py26: commands succeeded
  congratulations :)

$ find . -name 'std*_logfile'

[msabramo]

It's better to not create these files in the first place.

See #382

[msabramo]

#382 or something like it, where we prevent the files from being created in the first place is a better solution.