test: add feature_assumeutxo functional test

Most ideas for test improvements (TODOs) provided by Russ Yanofsky.

test/functional/test_framework/test_framework.py

@@ -979,3 +979,7 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
     def is_bdb_compiled(self):
         """Checks whether the wallet module was compiled with BDB support."""
         return self.config["components"].getboolean("USE_BDB")
+
+    def has_blockfile(self, node, filenum: str):
+        blocksdir = os.path.join(node.datadir, self.chain, 'blocks', '')
+        return os.path.isfile(os.path.join(blocksdir, f"blk{filenum}.dat"))