selftests: netfilter: Fix and review rpath.sh

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: 6e31ce83 ("selftests: netfilter: Test reverse path filtering")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>