python: Flush the simulation stdout/stderr buffers

python: Flush the simulation stdout/stderr buffers

Occasionally gem5's stdout/stderr, when run within the TestLib
framework, will be shuffled. This is resolved by flushing the
stdout/stderr buffer before and after simulation.

In addition to this, the verifier.py has been improved to remove
boilerplate gem5 code from the stdout comparison.

Change-Id: I04c8f9cee4475b8eab2f1ba9bb76bfa3cfcca6ec
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34995
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>