pytest: make test_pay_direct more effective.

The test sometimes passes: our routing logic always chooses between
the shorter of two equal-cost routes (because we compare best with <
not <=).

By adding another hop, we add more noise, and by making the alternate
route fee 0 we provide the worst case.

But to be fair, we make the amount of the payment ~50c (15,000,000
msat), and increase our cltv-delay to 14 and fee-base 1000 to match
mainnet.  The final patch shows the effect of this choice.

Otherwise our risk penalty is completely in the noise on
mainnet which has the vast majority of fees set at 1000msat + 1ppm.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_pay.py

@@ -1345,12 +1345,22 @@ def test_pay_routeboost(node_factory, bitcoind):
 def test_pay_direct(node_factory, bitcoind):
     """Check that we prefer the direct route.
     """
-    l1, l2, l3 = node_factory.get_nodes(3)
-
-    # Direct channel
+    # l2->l3 is really cheap by comparison.
+    l0, l1, l2, l3 = node_factory.get_nodes(4, opts=[{'fee-base': 1000,
+                                                      'cltv-delta': 14},
+                                                     {'fee-base': 1000,
+                                                      'cltv-delta': 14},
+                                                     {'fee-base': 0,
+                                                      'cltv-delta': 14},
+                                                     {'fee-base': 1000,
+                                                      'cltv-delta': 14}])
+
+    # Direct channel l0->l1->l3
+    l0.rpc.connect(l1.info['id'], 'localhost', l1.port)
+    l0.fund_channel(l1, 10**7)
     l1.rpc.connect(l3.info['id'], 'localhost', l3.port)
     l1.fund_channel(l3, 10**7)
-    # Indirect route
+    # Indirect route l0->l1->l2->l3
     l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
     l1.fund_channel(l2, 10**7)
     l2.rpc.connect(l3.info['id'], 'localhost', l3.port)
@@ -1359,8 +1369,8 @@ def test_pay_direct(node_factory, bitcoind):
     # Let channels lock in.
     bitcoind.generate_block(5)
 
-    # Make sure l1 knows the l2->l3 channel.
-    l1.wait_channel_active(c3)
+    # Make sure l0 knows the l2->l3 channel.
+    l0.wait_channel_active(c3)
 
     # Find out how much msatoshi l1 owns on l1->l2 channel.
     l1l2msatreference = only_one(l1.rpc.getpeer(l2.info['id'])['channels'])['msatoshi_to_us']
@@ -1368,9 +1378,9 @@ def test_pay_direct(node_factory, bitcoind):
     # Try multiple times to ensure that route randomization
     # will not override our preference for direct route.
     for i in range(8):
-        inv = l3.rpc.invoice(10000, 'pay{}'.format(i), 'desc')['bolt11']
+        inv = l3.rpc.invoice(15000000, 'pay{}'.format(i), 'desc')['bolt11']
 
-        l1.rpc.pay(inv)
+        l0.rpc.pay(inv)
 
         # We should have gone the direct route, so
         # l1->l2 channel msatoshi_to_us should not