tests: Update test scripts to work with Python 3

Change-Id: I71b1e595765fed9e9f234c9722c33ac5348d4f11
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15999
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>

tests,ext: Add a new testing library proposal

The new test library is split into two parts: The framework which resides
in ext/, and the gem5 helping components in /tests/gem5.

Change-Id: Ib4f3ae8d7eb96a7306335a3e739b7e8041aa99b9
Signed-off-by: Sean Wilson <spwilson2@wisc.edu>
Reviewed-on: https://gem5-review.googlesource.com/4421
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>