Changed python test suite to allow for shorter runs by default.

Changed python test suite to allow for shorter runs by default, with a --long (or -l) option for the complete runs.  Txn02 in particular now runs only a small number of scenarios by default.  Also added a `@longtest("description")` decorator for individual test methods. When a method is marked this way, it only runs  with the --long option.  Refs #1461.