tests/py: clean up set backend support fallout

tests/py: clean up set backend support fallout

Pablo reports failing py tests woth recent kernel and userland:
 any/objects.t: OK
WARNING: line 3: 'add rule ip6 test-ip6 input ..
mismatches 'family 2 __set0 test-ip4 3 backend nft_set_bitmap_type [nf_tables] count 7'

When nf_tables is built as a module, the set backend name coming
from kernel contains the module name ([nf_tables]), this makes the
test script treat it as part of the pseudo instructions.

Skip this line explicitly to avoid these warnings.

Fixes: 7cec20e45a75 ("tests/py: prepare for set debug change")
Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Tested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>