selftests: netfilter: conntrack_vrf.sh: move to lib.sh infra

swap test for "ip" with "conntrack", former is already accounted for
via setup_ns helper.  Also switch to bash.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-8-fw@strlen.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>