ext: Remove sandbox module from testlib

ext: Remove sandbox module from testlib

The sandbox module is providing a sandbox environment for
a specific TestCase via the multiprocessing package.

This isolation/complexity is not strictly needed as testlib is already
forking a new process via subprocess. As it is now, a TestRunner will
generate:

TestRunner -> multiprocessing.Process -> subprocess.Popen
(2 generated procs)

With this patch we are removing the intermediate layer

TestRunner -> subprocess.Popen
(1 generated proc)

JIRA: https://gem5.atlassian.net/projects/GEM5/issues/GEM5-533

Change-Id: Icd5cadbe316653a9269ab098ec4c07f21b864ad3
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30215
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>