testutils: add ip_ttl option to simple_*_packet
add test for the nx_dec_ttl vendor action

Based on a test by Gregor Maier.
testutils: fix wording in receive_pkt_check
nicira_dec_ttl: also test with a flow

Suggested by Gregor. The flow_match_test function doesn't support checking for
drops so I've kept the packet-out portion of the test too.