tests: add payer_note case for xpay

Author: Nazarevsky

tests/test_xpay.py

@@ -172,10 +172,10 @@ def test_xpay_simple(node_factory):
     b11 = l4.rpc.invoice('10000msat', 'test_xpay_simple', 'test_xpay_simple bolt11')['bolt11']
     l1.rpc.xpay(b11)
 
-    # BOLT 12.
+    # BOLT 12 (with payer_note specified).
     offer = l3.rpc.offer('any')['bolt12']
     b12 = l1.rpc.fetchinvoice(offer, '100000msat')['invoice']
-    l1.rpc.xpay(b12)
+    l1.rpc.xpay(invstring=b12, payer_note="Payment for super cheap coffee")
 
     # Failure from l4.
     b11 = l4.rpc.invoice('10000msat', 'test_xpay_simple2', 'test_xpay_simple2 bolt11')['bolt11']