tests: Update tests to save output on failure

tests: Update tests to save output on failure

The previous commit which tried to do this, did not work with parallel
execution. In this case, the fixtures that were modified were in the
child process and the parent process's fixtures were never updated.
Instead of modifying the object, use the information passed in from the
testlib.

See 4c28149ffa5d09e6fe14952dcaf8df5d0cd8f328
Previous review:
https://gem5-review.googlesource.com/c/public/gem5/+/17451

Change-Id: Ib4c06c5e3f82994199d6f0c1fa69452e93444d75
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19529
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>