Create a way to clean up tests output files upon success

Currently, a single build of HOMMEXX produces ~1.9GB of output files. On filesystem where multiple builds are present (i.e., fs shared among platforms, or where multiple branches are tested at the same time), this can make output files a burden.

Possible approach: add HOMME_CLEANUP_SUCCESSFUL_TESTS config option (default OFF); in diff-output.sh, before `exit 0`, if HOMME_CLEANUP_SUCCESSFUL_TESTS is ON, delete output nc files.

Second possible approach: create a target 'cleanUpTests', that the user can run manually if he needs to free up some space without deleting the build dir.

I vote for the first.

Start by reading diff-output.sh and the existing test configuration handling. Determine how the proposed HOMME_CLEANUP_SUCCESSFUL_TESTS option would default to OFF and identify the output .nc files removed after a successful test. Done means successful tests can clean up those files when enabled without deleting the build directory, while the default preserves current behavior.

Written by the indexing model from the issue text.