systemc: Add a --working-dir option to the test config.py.

systemc: Add a --working-dir option to the test config.py.

The tests expect to be run from a certain directory. Generally that
doesn't matter, but in at least one case the test opens a file with a
relative path, and that doesn't work unless CWD is what it expects.

Change-Id: I34c0ed975e77daed50ace4f7eebd034bf04c5595
Reviewed-on: https://gem5-review.googlesource.com/12271
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>