TST: updated meta tests for new behaviour
TST: updated meta tests for new behaviour

Updated the meta unit tests by:
- removing tests that do not test the Meta or MetaLabels classes,
- adding some docstrings,
- adding a common variable to setup,
- replacing old method calls with new method calls,
- fixing calls to MetaLabels, and
- using setup attributes throughout more unit tests.

rstoneback/pysat