Refactor getting Merkle root of data part of ByteList

tests/core/pyspec/eth2spec/test/helpers/custody.py

@@ -136,6 +136,12 @@ def build_proof(anchor, leaf_index):
     return list(reversed(proof))
 
 
+def get_block_data_merkle_root(data_as_bytelist):
+    # To get the Merkle root of the block data, we need the Merkle root without the length Mixing
+    # The below implements this in the Remerkleable framework
+    return data_as_bytelist.get_backing().get_left().merkle_root()
+
+
 def get_valid_custody_chunk_response(spec, state, chunk_challenge, block_length, challenge_index,
                                      invalid_chunk_data=False):
     custody_data = get_custody_test_vector(block_length)
@@ -160,8 +166,8 @@ def get_custody_test_vector(bytelength, offset=0):
 
 
 def get_shard_transition(spec, start_slot, block_lengths):
-    b = [ByteList[spec.MAX_SHARD_BLOCK_SIZE](get_custody_test_vector(x))
-         .get_backing().get_left().merkle_root() for x in block_lengths]
+    b = [get_block_data_merkle_root(ByteList[spec.MAX_SHARD_BLOCK_SIZE](get_custody_test_vector(x)))
+         for x in block_lengths]
     shard_transition = spec.ShardTransition(
         start_slot=start_slot,
         shard_block_lengths=block_lengths,
@@ -195,5 +201,5 @@ def get_custody_slashable_shard_transition(spec, start_slot, block_lengths, cust
     slashable_test_vector = get_custody_slashable_test_vector(spec, custody_secret,
                                                               block_lengths[0], slashable=slashable)
     block_data = ByteList[spec.MAX_SHARD_BLOCK_SIZE](slashable_test_vector)
-    shard_transition.shard_data_roots[0] = block_data.get_backing().get_left().merkle_root()
+    shard_transition.shard_data_roots[0] = get_block_data_merkle_root(block_data)
     return shard_transition, slashable_test_vector