test: refactor: replace unnecessary BytesIO uses

Rather than needing to create intermediate stream variables, we can use
helper functions like `tx_from_hex` instead or access the result
directly, leading both to increased readability and less code.

test/functional/interface_rest.py

@@ -7,9 +7,7 @@
 from decimal import Decimal
 from enum import Enum
 import http.client
-from io import BytesIO
 import json
-from struct import pack, unpack
 import typing
 import urllib.parse
 
@@ -160,12 +158,11 @@ class RESTTest (BitcoinTestFramework):
         bin_request = b'\x01\x02'
         for txid, n in [spending, spent]:
             bin_request += bytes.fromhex(txid)
-            bin_request += pack("i", n)
+            bin_request += n.to_bytes(4, 'little')
 
         bin_response = self.test_rest_request("/getutxos", http_method='POST', req_type=ReqType.BIN, body=bin_request, ret_type=RetType.BYTES)
-        output = BytesIO(bin_response)
-        chain_height, = unpack("<i", output.read(4))
-        response_hash = output.read(32)[::-1].hex()
+        chain_height = int.from_bytes(bin_response[0:4], 'little')
+        response_hash = bin_response[4:36][::-1].hex()
 
         assert_equal(bb_hash, response_hash)  # check if getutxo's chaintip during calculation was fine
         assert_equal(chain_height, 201)  # chain height must be 201 (pre-mined chain [200] + generated block [1])