tests: Specify a non-default root folder for regressions

tests: Specify a non-default root folder for regressions

The new testlib library is looking for regressions walking from
a root folder. This by default points to the tests dir.
Since all regressions are supposed to live in the tests/gem5 subdir,
the patch is assigning the gem5 subdir as a root directory.

This will prevent the example garbage to be printed in the ci framework:

Exception thrown while loading
"/tmpfs/src/git/jenkins-gem5-prod/tests/long/fs/10.linux-boot/test.py"
Ignoring all tests in this file.
Exception thrown while loading
"/tmpfs/src/git/jenkins-gem5-prod/tests/long/fs/80.solaris-boot/test.py"
Ignoring all tests in this file.
[...]

Change-Id: Ia12c6bbeda4ceac71ccd38156ab1e3bb98b05c89
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22726
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>