validation: fix feature_fastprune test - fund MiniWallet with UTXO

MiniWallet needs at least one UTXO before create_self_transfer() can be called.
Generate one block to fund the wallet before creating the large transaction.

This fixes the StopIteration error in CI where wallet.get_utxo() had no UTXOs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: claude

test/functional/feature_fastprune.py

@@ -24,6 +24,8 @@ def set_test_params(self):
     def run_test(self):
         self.log.info("ensure that large blocks don't crash or freeze in -fastprune")
         wallet = MiniWallet(self.nodes[0])
+        # Generate a block to fund the wallet with a UTXO
+        self.generate(wallet, 1)
 
         # Create a single transaction with large OP_RETURN to make block >64kb
         # We need to create a transaction that's large enough to exceed the fastprune limit