pyln-testing: add flag 'expect_fail' to node factory get_node

if the node fails to start (and we're expecting it to) return to us the
node object anyway

we also signal to collect all of its stderr logs by setting stderr
on the tailableproc that backs the node

contrib/pyln-testing/pyln/testing/utils.py

@@ -990,7 +990,7 @@ class NodeFactory(object):
 
     def get_node(self, node_id=None, options=None, dbfile=None,
                  feerates=(15000, 7500, 3750), start=True,
-                 wait_for_bitcoind_sync=True, **kwargs):
+                 wait_for_bitcoind_sync=True, expect_fail=False, **kwargs):
 
         node_id = self.get_node_id() if not node_id else node_id
         port = self.get_next_port()
@@ -1021,8 +1021,15 @@ class NodeFactory(object):
 
         if start:
             try:
-                node.start(wait_for_bitcoind_sync)
+                # Capture stderr if we're failing
+                if expect_fail:
+                    stderr = subprocess.PIPE
+                else:
+                    stderr = None
+                node.start(wait_for_bitcoind_sync, stderr=stderr)
             except Exception:
+                if expect_fail:
+                    return node
                 node.daemon.stop()
                 raise
         return node