Merge #19674: refactor: test: use throwaway _ variable for unused loop counters

dac7a111bd refactor: test: use _ variable for unused loop counters (Sebastian Falbesoner)

Pull request description:

  This tiny PR substitutes Python loops in the form of `for x in range(N): ...` by `for _ in range(N): ...` where applicable. The idea is indicating to the reader that a block (or statement, in list comprehensions) is just repeated N times, and that the loop counter is not used in the body, hence using the throwaway variable. This is already done quite often in the current tests (see e.g. `$ git grep "for _ in range("`). Another alternative would be using `itertools.repeat` (according to Python core developer Raymond Hettinger it's [even faster](https://twitter.com/raymondh/status/1144527183341375488)), but that doesn't seem to be widespread in use and I'm not sure about a readability increase.

  The only drawback I see is that whenever one wants to debug loop iterations, one would need to introduce a loop variable again. Reviewing this is basically a no-brainer, since tests would fail immediately if a a substitution has taken place on a loop where the variable is used.

  Instances to replace were found by `$ git grep "for.*in range("` and manually checked.

ACKs for top commit:
  darosior:
    ACK dac7a111bd
  instagibbs:
    manual inspection ACK dac7a111bd
  practicalswift:
    ACK dac7a111bd -- the updated code is easier to reason about since the throwaway nature of a variable is expressed explicitly (using the Pythonic `_` idiom) instead of implicitly. Explicit is better than implicit was we all know by now :)

Tree-SHA512: 5f43ded9ce14e5e00b3876ec445b90acda1842f813149ae7bafa93f3ac3d510bb778e2c701187fd2c73585e6b87797bb2d2987139bd1a9ba7d58775a59392406

test/functional/p2p_compactblocks.py

@@ -125,7 +125,7 @@ class CompactBlocksTest(BitcoinTestFramework):
         out_value = total_value // 10
         tx = CTransaction()
         tx.vin.append(CTxIn(COutPoint(block.vtx[0].sha256, 0), b''))
-        for i in range(10):
+        for _ in range(10):
             tx.vout.append(CTxOut(out_value, CScript([OP_TRUE])))
         tx.rehash()
 
@@ -266,7 +266,7 @@ class CompactBlocksTest(BitcoinTestFramework):
         address = node.getnewaddress()
 
         segwit_tx_generated = False
-        for i in range(num_transactions):
+        for _ in range(num_transactions):
             txid = node.sendtoaddress(address, 0.1)
             hex_tx = node.gettransaction(txid)["hex"]
             tx = FromHex(CTransaction(), hex_tx)
@@ -416,7 +416,7 @@ class CompactBlocksTest(BitcoinTestFramework):
     def build_block_with_transactions(self, node, utxo, num_transactions):
         block = self.build_block_on_tip(node)
 
-        for i in range(num_transactions):
+        for _ in range(num_transactions):
             tx = CTransaction()
             tx.vin.append(CTxIn(COutPoint(utxo[0], utxo[1]), b''))
             tx.vout.append(CTxOut(utxo[2] - 1000, CScript([OP_TRUE, OP_DROP] * 15 + [OP_TRUE])))
@@ -625,7 +625,7 @@ class CompactBlocksTest(BitcoinTestFramework):
         # Test that requesting old compactblocks doesn't work.
         MAX_CMPCTBLOCK_DEPTH = 5
         new_blocks = []
-        for i in range(MAX_CMPCTBLOCK_DEPTH + 1):
+        for _ in range(MAX_CMPCTBLOCK_DEPTH + 1):
             test_node.clear_block_announcement()
             new_blocks.append(node.generate(1)[0])
             wait_until(test_node.received_block_announcement, timeout=30, lock=mininode_lock)