Prior to this change, the test fails with KeyError if pong has
a zero value at the time this is called, as getpeerinfo's
bytesrecv_per_msg result excludes zero-values.

Combined these to a single wait_until as well, which will be a bit more
forgiving re the timeout while still enforcing the same 2 seconds
overall.

https://ci.appveyor.com/project/DrahtBot/bitcoin/builds/21310881#L62

test_runner.py rpc_net  # run a few times