Use make clean to reduce artifacts in successful jobs

Use make clean to reduce artifacts in successful jobs

Reduce the amount of artifacts stored by running make clean at the end
of unit and system test run. If any of the previous commands fail, the
runner will stop executing the commands in `script` immediately, so the
cleanup only happens if none of the previous commands failed.

The build artifacts from unit and system tests are re-used anywhere and
should be safe to throw away immediately. Same for respdiff.