tests/py: prepare for set debug change

tests/py: prepare for set debug change

Next patch will make initial set dump from kernel emit set debug
information, so the obtained netlink debug file won't match what is
recorded in tests/py.

Furthermore, as the python add rules for each of the family the test is
for, subsequent dump will include debug information of the other/previous
families.

Change the script to skip all unrelated information to only compare the
relevant set element information and the generated expressions.

This change still finds changes in [ expr ... ] and set elem debug output.

Signed-off-by: Florian Westphal <fw@strlen.de>