pytest: Added a simple breakpoint shorthand

I was typing this over and over again, adding this shorthand mainly to
remember it :-)

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

tests/test_lightningd.py

@@ -77,6 +77,8 @@ def setUpModule():
 def tearDownModule():
     tearDownBitcoind()
 
+def breakpoint():
+    import pdb; pdb.set_trace()
 
 class NodeFactory(object):
     """A factory to setup and start `lightningd` daemons.