Rename and clarify behavior of tests.py

tests.py attempts to install all datasets using all engines and logs errors. It does not use
a standard unit testing framework and it seems undesirable to regularly run this script as
part of testing.

1. It has been renamed it to reflect more clearly what it does and to prevent it from being
executed by nose.

2. The unused ``unittest`` import has been removed.

3. The docstring has been clarified.