Use distribution.targets instead of context.targets in decorators.py
Change ContextError to DistributionError in decorators.
Reflect changes in decorator tests
Fix regression when running on more than 4 engines.
Use ContextTestCase as base class.
Merge pull request #489 from enthought/bugfix/cluster-size-regression

Fixes regressions when running on a cluster with more or fewer than 4 engines