pytest: Stabilize test_setchannelfee_{restart,zero}

In both cases the flakyness arises from the destination not knowing about the
modified fees of the forwarding node, thus including the outdated details in
the routehint, and the sender being unlucky and always trying with the
routehint anyway.

The long-term solutions to this is going to be #4111, this commit just reduces
the flakyness to get back to business.

tests/test_pay.py

@@ -2034,6 +2034,10 @@ def test_setchannelfee_routing(node_factory, bitcoind):
     assert route[0]['msatoshi'] == 5002020
     assert route[1]['msatoshi'] == 4999999
 
+    # In case l3 includes a routehint, we need to make sure they also know
+    # about the new fees, otherwise we may end up with the old feerate
+    wait_for(lambda: [(c['base_fee_millisatoshi'], c['fee_per_millionth'], c['active']) for c in l3.rpc.listchannels(scid)['channels']] == [(1337, 137, True), (DEF_BASE, DEF_PPM, True)])
+
     # do and check actual payment
     inv = l3.rpc.invoice(4999999, 'test_setchannelfee_1', 'desc')['bolt11']
     result = l1.rpc.dev_pay(inv, use_shadow=False)
@@ -2054,6 +2058,10 @@ def test_setchannelfee_routing(node_factory, bitcoind):
     assert route[0]['msatoshi'] == 5000049
     assert route[1]['msatoshi'] == 4999999
 
+    # In case l3 includes a routehint, we need to make sure they also know
+    # about the new fees, otherwise we may end up with the old feerate
+    wait_for(lambda: [(c['base_fee_millisatoshi'], c['fee_per_millionth'], c['active']) for c in l3.rpc.listchannels(scid)['channels']] == [(DEF_BASE, DEF_PPM, True), (DEF_BASE, DEF_PPM, True)])
+
     # do and check actual payment
     inv = l3.rpc.invoice(4999999, 'test_setchannelfee_2', 'desc')['bolt11']
     result = l1.rpc.dev_pay(inv, use_shadow=False)
@@ -2091,6 +2099,10 @@ def test_setchannelfee_zero(node_factory, bitcoind):
     assert route[0]['msatoshi'] == 4999999
     assert route[1]['msatoshi'] == 4999999
 
+    # Wait for l3 to know about our low-balling, otherwise they'll add a wrong
+    # routehint to the invoice.
+    wait_for(lambda: [(c['base_fee_millisatoshi'], c['fee_per_millionth'], c['active']) for c in l3.rpc.listchannels(scid)['channels']] == [(0, 0, True), (DEF_BASE, DEF_PPM, True)])
+
     # do and check actual payment
     inv = l3.rpc.invoice(4999999, 'test_setchannelfee_3', 'desc')['bolt11']
     result = l1.rpc.dev_pay(inv, use_shadow=False)
@@ -2132,6 +2144,10 @@ def test_setchannelfee_restart(node_factory, bitcoind):
     # l1 wait for channel update from l2
     wait_for(lambda: [(c['base_fee_millisatoshi'], c['fee_per_millionth'], c['active']) for c in l1.rpc.listchannels(scid23)['channels']] == [(1337, 137, True), (DEF_BASE, DEF_PPM, True)])
 
+    # In case l3 includes a routehint, we need to make sure they also know
+    # about the new fees, otherwise we may end up with the old feerate
+    wait_for(lambda: [(c['base_fee_millisatoshi'], c['fee_per_millionth'], c['active']) for c in l3.rpc.listchannels(scid23)['channels']] == [(1337, 137, True), (DEF_BASE, DEF_PPM, True)])
+
     # l1 can make payment to l3 with custom fees being applied
     # Note: BOLT #7 math works out to 2021 msat fees
     inv = l3.rpc.invoice(4999999, 'test_setchannelfee_1', 'desc')['bolt11']