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_block.py

@@ -125,7 +125,7 @@ class FullBlockTest(BitcoinTestFramework):
 
         # collect spendable outputs now to avoid cluttering the code later on
         out = []
-        for i in range(NUM_OUTPUTS_TO_COLLECT):
+        for _ in range(NUM_OUTPUTS_TO_COLLECT):
             out.append(self.get_spendable_output())
 
         # Start by building a couple of blocks on top (which output is spent is