pytest: Change the channel persistence test to add inflight HTLCs

Now that we have HTLC persistence we'd also like to test it. This
kills the second node in the middle of an HTLC, it'll recover and
finish the flow.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

tests/utils.py

@@ -235,7 +235,7 @@ class LightningD(TailableProc):
         ]
 
         self.cmd_line += ["--{}={}".format(k, v) for k, v in LIGHTNINGD_CONFIG.items()]
-        self.prefix = 'lightningd'
+        self.prefix = 'lightningd(%d)' % (port)
 
         if not os.path.exists(lightning_dir):
             os.makedirs(lightning_dir)