tests: Always print stderr in gem5 Fixtures

tests: Always print stderr in gem5 Fixtures

At the moment is impossible when observing an upstream kokoro failure
to understand what went wrong.
This is because the only thing that gets printed is the exception
traceback and the command line generating it.

Most of the time it will be something like

Traceback (most recent call last):
  File
"/tmpfs/src/git/jenkins-gem5-prod/tests/../ext/testlib/runner.py", line
195, in setup
    fixture.setup(testitem)
  File "/tmpfs/src/git/jenkins-gem5-prod/tests/gem5/fixture.py", line
115, in setup
    self._setup(testitem)
  File "/tmpfs/src/git/jenkins-gem5-prod/tests/gem5/fixture.py", line
160, in _setup
    log_call(log.test_log, command)
  File
"/tmpfs/src/git/jenkins-gem5-prod/tests/../ext/testlib/helper.py", line
103, in log_call
    raise subprocess.CalledProcessError(retval, cmdstr)

With this patch we dump the stderr so that the fail reason is exposed
to the viewer.

Change-Id: Ic3d0fe75ec4d0543d95e9624dc5287afb4af3b8b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23843
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>