systemc: Run the systemc tests with the CWD in the build directory.

systemc: Run the systemc tests with the CWD in the build directory.

This will prevent some (but not all) output files the tests generate
from ending up outside the build directory. Because some output file
streams are constructed as global objects, their paths are resolved
relative to the CWD when gem5 starts, before the config script has a
chance to change it.

Subsequent changes will make verify.py should make gem5 start with the
correct working directory, cleaning up the remaining leaking files.

Change-Id: I75a1256719dab4c98ab868c209d09b9dcdabb458
Reviewed-on: https://gem5-review.googlesource.com/c/14518
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>