ext: Fix tmpfiles in testlib

ext: Fix tmpfiles in testlib

Previously, the testlib would generate and not clean up 100s of
temporary files in /tmp. This puts all of the tempfiles in the same
directory to make sure they are cleaned up on a successful test.

Change-Id: If4dcd93ab4b5df556c046753266a71365d1388c1
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19528
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>