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/test_framework/script.py

@@ -646,7 +646,7 @@ def LegacySignatureHash(script, txTo, inIdx, hashtype):
 
         tmp = txtmp.vout[outIdx]
         txtmp.vout = []
-        for i in range(outIdx):
+        for _ in range(outIdx):
             txtmp.vout.append(CTxOut(-1))
         txtmp.vout.append(tmp)