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/wallet_multiwallet.py

@@ -26,7 +26,7 @@ FEATURE_LATEST = 169900
 got_loading_error = False
 def test_load_unload(node, name):
     global got_loading_error
-    for i in range(10):
+    for _ in range(10):
         if got_loading_error:
             return
         try: