ext: Make the testing-results folder visible

ext: Make the testing-results folder visible

Currently, testlib will write outputs to .testing-results, which
is a hidden folder. However, the outputs in the folder are useful
for debugging. This commit makes this folder visible on Linux
systems.

Change-Id: I158e1a4a62c7b494309a0a4bf6521fedd04cbe22
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34015
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>