[tests] p2p_segwit: wrap subtests with subtest wrapper.
[tests] p2p_segwit: wrap subtests with subtest wrapper.

The subtest wrapper logs the name of the subtest.

chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin