Address a few problems with the initial test script version:
* On systems with ip6tables but no ip6tables-legacy, testing for
ip6tables was disabled by accident.
* Firewall setup phase did not respect possibly unavailable tools.
* Consistently call nft via '$nft'.
Fixes: 6e31ce831c ("selftests: netfilter: Test reverse path filtering")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
58bb78ce02