tests/plugintest.py - Prevent unittests from spamming stdout

Previously when unittests on pynag.Plugins where being run, there would
be a lot of output going to stdout and stderr.

This patch disables stdout and stderr while running the tests.
Merge pull request #105 from pynag/unittests-supress-stdout

tests/plugintest.py - Prevent unittests from spamming stdout