refactor: test: use _ variable for unused loop counters

substitutes "for x in range(N):" by "for _ in range(N):"
indicates to the reader that a block is just repeated N times, and
that the loop counter is not used in the body

test/functional/feature_rbf.py

@@ -376,7 +376,7 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         split_value = int((initial_nValue-fee)/(MAX_REPLACEMENT_LIMIT+1))
 
         outputs = []
-        for i in range(MAX_REPLACEMENT_LIMIT+1):
+        for _ in range(MAX_REPLACEMENT_LIMIT+1):
             outputs.append(CTxOut(split_value, CScript([1])))
 
         splitting_tx = CTransaction()