Fix #555. Move the set of DRUSH_BACKEND context from _drush_bootstrap_output_prepare() to _drush_bootstrap_global_options().
Move from using php-eval to core-script for easier debugging in testDispatchUsingAlias.
Pass along return value in core-script command.
Skip testDispatchUsingAlias for now as it is failing. I think the test is flawed, not the functionality.