test_lightningd.py: Test 0-msatoshi pay

Based-on-patch-by: ZmnSCPxj <ZmnSCPxj@protonmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_lightningd.py

@@ -806,6 +806,9 @@ class LightningDTests(BaseLightningDTests):
             # Must provide an amount!
             self.assertRaises(ValueError, l1.rpc.pay, inv2)
             self.assertRaises(ValueError, l1.rpc.pay, inv2, None)
+            # Amount must be nonzero!
+            self.assertRaisesRegex(ValueError, 'WIRE_AMOUNT_BELOW_MINIMUM',
+                                   l1.rpc.pay, inv2, 0)
             l1.rpc.pay(inv2, random.randint(1000, 999999))
 
         # Should see 6 completed payments