tests: shell: Fix ifname_based_hooks feature check

tests: shell: Fix ifname_based_hooks feature check

The test was technically incorrect: Instead of detecting whether
interface hooks are name-based or not, it actually tested whether
netdev-family chains are removed along with their last hook.

Since the latter behaviour is established in kernel commit fc0133428e7a
("netfilter: nf_tables: Tolerate chains with no remaining hooks") and
thus independent from the name-based hooks change, treating both as the
same kernel feature is not acceptable.

Fix this by detecting whether a netdev-family chain may be added despite
specifying a non-existent interface to hook into. Keep the old check
around with a better name, although unused for now.

Reported-by: Florian Westphal <fw@strlen.de>
Fixes: f27e5abd81f29 ("tests: shell: Adjust to ifname-based hooks")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>