systemc: Record the test's return code in the output directory.

systemc: Record the test's return code in the output directory.

verify.py will be able to use that value later to determine whether a
test was successful.

Change-Id: Ib499a4b6f202ee46e66e5de1b61a7e6309df9886
Reviewed-on: https://gem5-review.googlesource.com/12052
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>