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_assumevalid.py

@@ -123,7 +123,7 @@ class AssumeValidTest(BitcoinTestFramework):
         height += 1
 
         # Bury the block 100 deep so the coinbase output is spendable
-        for i in range(100):
+        for _ in range(100):
             block = create_block(self.tip, create_coinbase(height), self.block_time)
             block.solve()
             self.blocks.append(block)
@@ -149,7 +149,7 @@ class AssumeValidTest(BitcoinTestFramework):
         height += 1
 
         # Bury the assumed valid block 2100 deep
-        for i in range(2100):
+        for _ in range(2100):
             block = create_block(self.tip, create_coinbase(height), self.block_time)
             block.nVersion = 4
             block.solve()