util,tests: Added .txt file extension to txt files

util,tests: Added .txt file extension to txt files

This is a small improvement. In our Jenkins, https://jenkins.gem5.org,
we archive the `compile-test-out` directory. Opening these `*.stderr`
and `*.stdout` files through the Jenkins interface was problematic. The
`.txt` extension makes these files easier to open.

Change-Id: I4026efec2118179eaed775c7560510cd16f349a6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32154
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>