Merge #12545: test: Use wait_until to ensure ping goes out

Summary:
0eb84f30d8 test: Use wait_until to ensure ping goes out (Ben Woosley)

Pull request description:

Intermittent failure evident here:
https://travis-ci.org/bitcoin/bitcoin/jobs/344021180

Tree-SHA512: 8d3cbc55563160390d3918eb0b22348990cd81ad35c0563033f19142cb43b86a150fecc7df2196209ba00aaa805c67285451e582f387f8dac22d5e44d54e8a0d

Backport of Core PR12545
https://github.com/bitcoin/bitcoin/pull/12545/files

Test Plan: test_runner.py rpc_net

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3733