unit_test_executor: Propagate exit codes correctly

Several changes to ensure that exit codes of the main test executable (e.g. athena) get propagated correctly:

• Remove set -e to ensure that the post-processing runs even if the test executable failed.
• Remove unnecessary check if test executable succeeded.
• Set a default return code before executing a potential post-processing script. The post-processing script then has the possibility to override this if needed.