Improve code coverage by adding tests

Original LP Author: Terri , LP Link: https://bugs.launchpad.net/bugs/1414352

Mailman Core and Postorius have sets of tests, but could use more extensive test coverage. This issue is meant to attract new contributors (e.g. prospective Google Summer of Code Students). In short: find a piece of Postorius or Mailman Core that isn't yet tested and write a test (or set of tests) for it.

Please do not discuss the code used to implement tests here. You can ask about how to get started, what extra infrastructure you need (tox, nose, nose2, unittest, ...) etc, but once you actually start creating test cases you should open a separate issue for that test or set of related tests. Also, if you have to come here to find information, that's probably a documentation bug.

Contributors wishing to work on Mailman Core should look at START.rst and DATABASE.rst for more information about setting up tests.

For Postorius, Florian is working on vcrpy test integration, so new tests can use that format.

Links to documentation and examples much appreciated to update this bug!