test: functional test for new coin selection logic

Create a wallet with mixed OutputTypes and send a volley of payments,
ensuring that there are no mixed OutputTypes in the txs. Finally,
verify that OutputTypes are mixed only when necessary.

test/functional/test_runner.py

@@ -156,6 +156,7 @@ BASE_SCRIPTS = [
     'mempool_spend_coinbase.py',
     'wallet_avoidreuse.py --legacy-wallet',
     'wallet_avoidreuse.py --descriptors',
+    'wallet_avoid_mixing_output_types.py --descriptors',
     'mempool_reorg.py',
     'mempool_persist.py',
     'p2p_block_sync.py',