systemc: Run the systemc tests directly from their build directories.

systemc: Run the systemc tests directly from their build directories.

We were previously running them from the current directory to start
with, and then having the config script switch to the build directory.
That worked, except when output streams might be opened as part of the
global constructors which would run before the config script.

This change makes us start from the build directory directly, making
the switch in the config script unnecessary and ensuring that no files
leak outside of the build when running tests.

Change-Id: I484168793bfc5abc4e5631fb3468733fb9d829af
Reviewed-on: https://gem5-review.googlesource.com/c/14519
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>