ext: Add support for comma-separated inputs of testlib argparser

ext: Add support for comma-separated inputs of testlib argparser

Currently, the --isa, --variant and --length options of testlib
do not support comma-separated inputs. This commit adds the
support for such an input for those options.

The argument parser now supports specifying the parameters multiple
times as well as specifying multiple options at a time.

JIRA: https://gem5.atlassian.net/jira/software/c/projects/GEM5/issues/GEM5-770

Change-Id: I3c276a9b9d9c6b0b802ecf8e7f1f9a3dfafe45d1
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34198
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>