tests/py: Set a fixed timezone in nft-test.py

tests/py: Set a fixed timezone in nft-test.py

Payload generated for 'meta time' matches depends on host's timezone and
DST setting. To produce constant output, set a fixed timezone in
nft-test.py. Choose UTC-2 since most payloads are correct then, adjust
the remaining two tests.

Fixes: 0518ea3f70d8c ("tests: add meta time test cases")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Ander Juaristi <a@juaristi.eus>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>