cassini-[ci,config]: Collate test results in junit format

Collate test results in junit format from all the child pipeline jobs
and upload them as artifact so that the test summary can be seen in
`Overview` section of Merge Requests.

The script `.gitlab/scripts/child-job-downloader.py` is based on a
third-party tool. Therefore, exclude it from qa python check.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>