[9.20] fix: test: enable shell-based rndc system tests

Enable existing rndc system tests (the python test function calling the
shell file was missing). Also update the extra artifacts list to remove
one generated file which was left behind.

Backport of MR !10489

Merge branch 'backport-colin/rndc-tests-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10493