cleanups to the existing test suite

This adds no new behavior.  These are cleanups to the test suite
to get it up to modern Assert testing style/standards.

In the process of cleaning up, I noticed the file store tests were
not very robust.  I updated the tests and in the process changed
the internal API for accessing a "null store" object.
Merge pull request #64 from redding/kr-test-cleanups

cleanups to the existing test suite