message: test: refactor: simplify `block_submit` in feature_nulldummy.py
  The `create_block` helper accepts a list of txs that it includes in the
  created block, hence we don't have to do that manually. Also, rehashing
  before solving the block is not needed and can be removed.
diff: @@ -121,11 +121,7 @@ class NULLDUMMYTest(BitcoinTestFramework):
  tmpl = node.getblocktemplate(NORMAL_GBT_REQUEST_PARAMS)
  assert_equal(tmpl['previousblockhash'], self.lastblockhash)
  assert_equal(tmpl['height'], self.lastblockheight + 1)
  - block = create_block(tmpl=tmpl, ntime=self.lastblocktime + 1)
  - for tx in txs:
  - tx.rehash()
  - block.vtx.append(tx)
  - block.hashMerkleRoot = block.calc_merkle_root()
  + block = create_block(tmpl=tmpl, ntime=self.lastblocktime + 1, txlist=txs)
  if with_witness:
  add_witness_commitment(block)
  block.solve()
  