tests: fixe test_multichan_stress flake

```
         def send_many_payments():
             passes = 0
             fails = 0
             # Make sure we try many times, and get at least one pass and fail.
             while passes == 0 or fails == 0 or passes + fails < 30:
                 inv = l3.rpc.invoice(100, "label-" + str(passes + fails), "desc")
                 l1.rpc.sendpay(route, inv['payment_hash'], payment_secret=inv['payment_secret'])
                 time.sleep(0.05)
                 try:
                     l1.rpc.waitsendpay(inv['payment_hash'])
                     passes += 1
                 except RpcError:
                     fails += 1
                     pass

         # Send a heap of payments, while reconnecting...
         fut = executor.submit(send_many_payments)

         for _ in range(30):
             l3.rpc.disconnect(l2.info['id'], force=True)
             time.sleep(0.1)
             l3.rpc.connect(l2.info['id'], 'localhost', l2.port)
 >       fut.result(TIMEOUT)
```

The while loop would somtimes go infinitely long if no payments fail.

Changelog-None

tests/test_connection.py

@@ -3868,18 +3868,18 @@ def test_multichan_stress(node_factory, executor, bitcoind):
               'channel': scid23}]
 
     def send_many_payments():
-        passes = 0
-        fails = 0
-        # Make sure we try many times, and get at least one pass and fail.
-        while passes == 0 or fails == 0 or passes + fails < 30:
-            inv = l3.rpc.invoice(100, "label-" + str(passes + fails), "desc")
+        # Whether each payment succeeds or fails depends on whether the
+        # l2-l3 connection happens to be up when the HTLC is forwarded
+        # (reconnects are fast), so we can't rely on getting either.
+        # Send a bounded number while the reconnects run, and let the
+        # payments below verify the channels work afterwards.
+        for i in range(50):
+            inv = l3.rpc.invoice(100, "label-" + str(i), "desc")
             l1.rpc.sendpay(route, inv['payment_hash'], payment_secret=inv['payment_secret'])
             time.sleep(0.05)
             try:
                 l1.rpc.waitsendpay(inv['payment_hash'])
-                passes += 1
             except RpcError:
-                fails += 1
                 pass
 
     # Send a heap of payments, while reconnecting...