test-fixtures: return error set, don't throw exception

Throwing an exception while killing all nodes meant that
we aren't cleaning up all the nodes properly. Instead,
collect the errors, and return them back to the upper level,
where we report them and terminate as expected.

tests/utils.py

@@ -967,6 +967,7 @@ class NodeFactory(object):
     def killall(self, expected_successes):
         """Returns true if every node we expected to succeed actually succeeded"""
         unexpected_fail = False
+        err_msgs = []
         for i in range(len(self.nodes)):
             leaks = None
             # leak detection upsets VALGRIND by reading uninitialized mem.
@@ -985,9 +986,10 @@ class NodeFactory(object):
                     unexpected_fail = True
 
             if leaks is not None and len(leaks) != 0:
-                raise Exception("Node {} has memory leaks: {}".format(
+                unexpected_fail = True
+                err_msgs.append("Node {} has memory leaks: {}".format(
                     self.nodes[i].daemon.lightning_dir,
                     json.dumps(leaks, sort_keys=True, indent=4)
                 ))
 
-        return not unexpected_fail
+        return not unexpected_fail, err_msgs