pytest: simple test for pay's custom notifications.

diff --git a/tests/test_plugin.py b/tests/test_plugin.py
index d1c6f293..9abe7c60 100644
--- a/tests/test_plugin.py
+++ b/tests/test_plugin.py
@@ -10,7 +10,7 @@ from utils import (
     expected_peer_features, expected_node_features,
     expected_channel_features, account_balance,
     check_coin_moves, first_channel_id, EXPERIMENTAL_DUAL_FUND,
-    mine_funding_to_announce, VALGRIND
+    mine_funding_to_announce, VALGRIND, first_scid
 )
 
 import ast
@@ -4374,6 +4374,70 @@ def test_peer_storage(node_factory, bitcoind):
     assert not l2.daemon.is_in_log(r'PeerStorageFailed')
 
 
+def test_pay_plugin_notifications(node_factory, bitcoind, chainparams):
+    plugin = os.path.join(os.getcwd(), 'tests/plugins/all_notifications.py')
+    opts = {"plugin": plugin}
+
+    l1, l2, l3 = node_factory.line_graph(3, opts=[opts, {}, {}],
+                                         wait_for_announce=True)
+
+    def zero_timestamps(obj):
+        if isinstance(obj, dict):
+            for k, v in obj.items():
+                if k == "timestamp":
+                    obj[k] = 0
+                else:
+                    zero_timestamps(v)
+        elif isinstance(obj, list):
+            for item in obj:
+                zero_timestamps(item)
+        # other types are ignored
+        return obj
+
+    inv1 = l3.rpc.invoice(20000, "first", "desc")
+    l1.rpc.pay(inv1['bolt11'])
+
+    # It gets a channel hint update notification
+    line = l1.daemon.wait_for_log(f"plugin-all_notifications.py: notification channel_hint_update: ")
+    dict_str = line.split("notification channel_hint_update: ", 1)[1]
+    data = zero_timestamps(ast.literal_eval(dict_str))
+
+    # pyln-client's plugin.py duplicated payload into same name as update.
+    channel_hint_update_core = {'scid': first_scid(l1, l2) + '/1',
+                                'estimated_capacity_msat': 964719000 if chainparams['elements'] else 978718000,
+                                'total_capacity_msat': 1000000000,
+                                'timestamp': 0,
+                                'enabled': True}
+    channel_hint_update = {'origin': 'pay',
+                           'payload': {'channel_hint': channel_hint_update_core}}
+    assert data == channel_hint_update
+
+    # It gets a success notification
+    line = l1.daemon.wait_for_log(f"plugin-all_notifications.py: notification pay_success: ")
+    dict_str = line.split("notification pay_success: ", 1)[1]
+    data = ast.literal_eval(dict_str)
+    success_core = {'payment_hash': inv1['payment_hash'],
+                    'bolt11': inv1['bolt11']}
+    # Includes deprecated and modern.  pyln-client plugin.py copies fields as necessary.
+    success = {'origin': 'pay',
+               'payload': success_core}
+    assert data == success
+
+    inv2 = l3.rpc.invoice(10000, "second", "desc")
+    l3.rpc.delinvoice('second', 'unpaid')
+    with pytest.raises(RpcError, match="WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS"):
+        l1.rpc.pay(inv2['bolt11'])
+
+    line = l1.daemon.wait_for_log(f"plugin-all_notifications.py: notification pay_failure: ")
+    dict_str = line.split("notification pay_failure: ", 1)[1]
+    data = ast.literal_eval(dict_str)
+    failure_core = {'payment_hash': inv2['payment_hash'], 'bolt11': inv2['bolt11'], 'error': {'message': 'failed: WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS (reply from remote)'}}
+    # Includes deprecated and modern.
+    failure = {'origin': 'pay',
+               'payload': failure_core}
+    assert data == failure
+
+
 @pytest.mark.openchannel('v1')
 @pytest.mark.openchannel('v2')
 def test_openchannel_hook_channel_type(node_factory, bitcoind):