test_lightning.py: shutdown tail thread before killing.

Seems to avoid the nasty python resource warnings, as well as the
fatal 'ValueError: PyMemoryView_FromBuffer(): info->buf must not be NULL'

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

tests/utils.py

@@ -85,6 +85,12 @@ class TailableProc(object):
 
         return self.proc.returncode
 
+    def kill(self):
+        """Kill process without giving it warning."""
+        self.proc.kill()
+        self.proc.wait()
+        self.thread.join()
+    
     def tail(self):
         """Tail the stdout of the process and remember it.