pyln: add context manager to simpify filter use.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: pyln: LightningRpc has new `reply_filter` context manager for reducing output of RPC commands.

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

@@ -705,7 +705,8 @@ class PrettyPrintingLightningRpc(LightningRpc):
             "result": res
         }, indent=2))
 
-        if schemas and schemas[1] and not filter:
+        # FIXME: if filter set, just remove "required" from schemas?
+        if schemas and schemas[1] and filter is None and self._filter is None:
             schemas[1].validate(res)
 
         return res