Refs #8315 - Simplify ADS cleanup after each test.

Replaced multiple calls to ADS.remove() with a single tearDown fixture.
Refs #8315 - Clean up after multifile loading.

* Remove temp workspace at end of summing.
* Ensure output of load is in the workspace it's supposed to be.
Merge branch 'bugfix/8315_fix_excess_group_when_loading_summed_ws' into develop